215
REVIEW ARTICLE
Possible use of antimicrobial agents in cancer therapy
Copyright (c) 2022 Slovenian Medical Journal. This work is licensed under a
Creative Commons Attribution-NonCommercial 4.0 International License.
Possible use of antimicrobial agents in cancer therapy
Možnosti uporabe protimikrobnih učinkovin pri zdravljenju raka
Ana Mitrović,1 Janko Kos1,2
Abstract
Cancer represents a major burden for health systems and is the second leading cause of death in the developed world. 
Despite major progress during past years, a lot of resources are still directed towards the identification of new antitumour 
drugs that would allow more efficient treatment, and prolonged patients’ survival, lower cancer recurrence rate, and de-
creased side effects associated with cancer therapy, thus increased quality of life for cancer patients. However, the intro-
duction of new drugs to clinical practice is associated with high costs. To keep the health system sustainable, the search for 
new indications of existing drugs has attracted a lot of attention in the field of oncology. Among others, antitumour activity 
has been shown for some antibiotics and other antimicrobial agents, including doxycycline, chloroquine, nitroxoline, and 
certain fluoroquinolones. In this review, we summarize various molecular mechanisms and tumour models used to define 
their antitumour activity. The latter has been demonstrated in a number of independent studies both in vitro as well as in 
vivo. Moreover, structures of existing compounds were used as lead compounds for the development of new derivatives 
and the identification of structural elements that improve antitumour activity. Together, these studies point to new possi-
bilities for the use of already well-established drugs, which could expand their range, and, either, alone or in combination 
with existing therapy, improve the effectiveness of antitumour therapy.
Izvleček
Rak je veliko breme za zdravstvene sisteme in še vedno drugi najpogostejši vzrok smrti v razvitem svetu. Zato se, kljub 
velikem napredku v zadnjih letih, še vedno veliko sredstev usmerja v iskanje novih učinkovitejših protitumorskih zdravil, 
ki bi omogočila bolj učinkovito zdravljenje, daljše preživetje bolnikov z rakom, preprečila ponovni pojav raka in zmanjšala 
stranske učinke, povezane z zdravljenjem raka, s tem pa povečala kakovost življenja bolnikov z rakom. Uvajanje novih 
zdravil v klinično uporabo pa je hkrati povezano z vse večjimi stroški. Zato je zaradi velike potrebe po prepoznavanju novih 
protitumorskih zdravil iskanje novih indikacij za že obstoječa zdravila na področju onkologije pritegnilo veliko pozornosti. 
Pri tem se je med drugim protitumorsko delovanje pokazalo tudi za nekatere antibiotike in druge protimikrobne učinko-
vine. Slednje se je med drugim izkazalo za doksiciklin, klorokin, nitroksolin in nekatere predstavnike fluorokinolonov. V 
1 Department of Biotechnology, Jožef Stefan Institute, Ljubljana, Slovenia
2 Faculty of Pharmacy, University of Ljubljana, Ljubljana, Slovenia
Correspondence / Korespondenca: Ana Mitrović, e: ana.mitrovic@ijs.si
Key words: drug repurposing; antitumour therapy; antimicrobial agents; cancer; drug discovery
Ključne besede: premeščanje zdravil; protitumorska terapija; protimikrobne učinkovine; rak; iskanje novih učinkovin
Received / Prispelo: 2. 11. 2020 | Accepted / Sprejeto: 16. 3. 2021
Cite as / Citirajte kot: Mitrović A, Kos J. Possible use of antimicrobial agents in cancer therapy. Zdrav Vestn. 2022;91(5–6):215–25. DOI: 
https://doi.org/10.6016/ZdravVestn.3188
eng slo element
en article-lang
10.6016/ZdravVestn.3188 doi
2.11.2020 date-received
16.3.2021 date-accepted
Cytology, oncology, cancerology Citologija, onkologija, kancerologija discipline
Review article Pregledni znanstveni članek article-type
Possible use of antimicrobial agents in cancer 
therapy
Možnosti uporabe protimikrobnih učinkovin pri 
zdravljenju raka article-title
Possible use of antimicrobial agents in cancer 
therapy
Možnosti uporabe protimikrobnih učinkovin pri 
zdravljenju raka alt-title
drug repurposing, antitumour therapy, antimi-
crobial agents, cancer, drug discovery
premeščanje zdravil, protitumorska terapija, 
protimikrobne učinkovine, rak, iskanje novih 
učinkovin
kwd-group
The authors declare that there are no conflicts 
of interest present.
Avtorji so izjavili, da ne obstajajo nobeni 
konkurenčni interesi. conflict
year volume first month last month first page last page
2022 91 5 6 215 225
name surname aff email
Ana Mitrović 1 ana.mitrovic@ijs.si
name surname aff
Janko Kos 1,2
eng slo aff-id
Department of Biotechnology, 
Jožef Stefan Institute, Ljubljana, 
Slovenia
Odsek za biotehnologijo, Inštitut 
Jožef Stefan, Ljubljana, Slovenija 1
Faculty of Pharmacy, University 
of Ljubljana, Ljubljana, Slovenia
Fakulteta za farmacijo, Univerza v 
Ljubljani, Ljubljana, Slovenija 2
Slovenian Medical Journallovenian Medical Journal
216
CYTOLOGY, ONCOLOGY, CANCEROLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3188
1 Introduction
After cardiovascular diseases, cancer is one of the 
biggest health problems and the second leading cause of 
death in the developed world, including Slovenia. Ac-
cording to the World Health Organization (WHO), 9.6 
million people died of cancer worldwide in 2018 (1). In 
addition to being a major health problem, cancer also 
presents a major social and economic burden in today’s 
society (1). Despite major advances in cancer treatment 
in recent years, we still face many challenges (2). The key 
challenges in developing more effective therapies are to 
prevent cancer recurrence, avoid therapy resistance and 
reduce the many side-effects associated with most of the 
therapeutic approaches currently used in clinical prac-
tice, which further reduce the quality of life for cancer 
patients (3). The development of new drugs is extreme-
ly costly and the costs only continue to increase, which 
is why the number of new drugs on the market has de-
clined over the last decade, despite high investment in 
research and development (4).
In response to these high costs and the small num-
ber of drugs that successfully pass all stages of testing 
and reach the market, more and more attention is being 
dedicated to finding new indications and uses for drugs 
already on the market (so-called drug repurposing, or 
repositioning) (5,6). The toxicological profiles, tolerabil-
ity, pharmacokinetic and pharmacodynamic properties 
of these drugs are already well known, and these are the 
main advantages that increase their potential for use in 
cancer treatment. For existing drugs, their dosage regi-
men, pharmacological properties and their interactions 
with other drugs are known (2,5-7). Due to the large 
amount of known data, the use of existing active sub-
stances for new indications is therefore associated with 
shorter time and lower costs required for the substances’ 
successful translation into clinical use (2,5,6).
When looking for new indications for existing drugs, 
the first step is to investigate in detail their mechanism 
of action on new targets and to test their efficacy and 
safety in clinical studies, as the use of a drug for a new 
indication may be associated with new side-effects that 
are not yet known (5,6). However, for existing drugs, it 
preglednem članku povzemamo različne molekulske mehanizme, uporabljene tumorske modele in raziskave, ki oprede-
ljujejo njihovo protitumorsko delovanje. To so prikazale številne neodvisne študije tako in vitro kot in vivo, strukture ob-
stoječih učinkovin pa so se hkrati uporabile kot spojine za razvoj novih derivatov in identificiranje strukturnih elementov, 
ki izboljšajo protitumorsko delovanje. Te raziskave tako kažejo na nove možnosti za uporabo že dobro znanih učinkovin, 
ki bi lahko razširile svoj spekter uporabe in samostojno ali v kombinaciji z že obstoječim zdravljenjem izboljšale uspešnost 
protitumorskega zdravljenja.
is possible to move more quickly into phases 2 and 3 of 
clinical trials due to the large amount of information al-
ready obtained in previous pre-clinical and clinical trials 
(2). In addition to drugs that are in clinical use, active 
substances that have been shown to be safe in clinical 
trials but have not entered clinical use for other reasons 
are also well suited to the search for new indications (6).
The drug repurposing has attracted particular atten-
tion in the field of oncology, due to the great need to 
identify new, more effective active substances (3,5). It is 
now known that many active substances that are used to 
treat other indications and differ in both their structure 
and mechanism of action, can also show antitumour ac-
tivity. Antitumour activity has already been confirmed 
for thalidomide, metformin, acetylsalicylic acid, statins, 
raloxifene, tamoxifen, certain antidepressants and anti-
psychotics, and many other drugs (2,5).
In addition to these, some antibiotics and antimi-
crobials have also shown antitumour activity, including 
doxycycline, chloroquine, nitroxoline and fluroquinolo-
nes (Figure 1). In this review article, we will focus on 
the potential use of some antimicrobial agents for cancer 
treatment and present possible mechanisms for their an-
titumour activity.
2 Doxycycline
Doxycycline (Figure 1) is a tetracycline antibiotic 
used to treat a wide range of bacterial infections (5,8). It 
is also used prophylactically in malaria prevention and is 
effective in the treatment of malaria in combination with 
quinine. Doxycycline is rapidly absorbed and rapidly 
crosses the blood-brain barrier following oral adminis-
tration, its pharmacokinetic properties and half-life are 
known, and it is generally well tolerated by patients fol-
lowing administration (8).
In recent years, several studies have shown that, in 
addition to its potent antibacterial activity, doxycycline 
has antitumour activity. Antitumour activity of doxycy-
cline has been demonstrated in various types of cancer, 
including breast, cervical, ovarian, prostate, lung, oral, 
217
REVIEW ARTICLE
Possible use of antimicrobial agents in cancer therapy
Figure 1: Structures of antimicrobial agents exhibiting antitumour activity.
N
OH
NO 2
OH O OH O O
NH 2
OH
H
OH N
OH
H
NCl
HN
N
X NR 7
F
R 5 O
OH
O
R 2
R 1
NN
F
O
OH
O
HN
NN
F
O
OH
O
HN F
NH 2
N NN
F
O
OH
O
H2N
N
O
N NN
F
O
OH
O
HN
Doxycycline
Fluoroquinolones
Chloroquine Nitroxoline
General structure of 
fluoroquinolones
Ciprofloxacin
Enoxacin
Gemifloxacin
Sparfloxacin
Selected fluoroquinolones with antitumor activity
pancreatic and duodenal cancer, colorectal cancer, mel-
anoma and leukaemia (5,8). More than 30 years ago, in 
leukaemia cells, doxycycline treatment was shown to 
inhibit T-cell proliferation and lead to their complete 
destruction, indicating that the mechanism of action of 
doxycycline depends on its concentration and the de-
gree of tumour progression (9).
Several mechanisms by which doxycycline inhibits 
tumour formation and progression have been described 
(8). As one of its mechanisms of action, doxycycline in-
hibits the matrix metallopeptidases MMP-2 and MMP-
9, which are important players in various processes of 
cancer onset and progression. It further inhibits MMP 
activity by increasing the expression of tissue inhibitor 
MMP-2 (TIMP-2) (5,10-12). In animal models, doxy-
cycline has been shown to inhibit metastases by inhib-
iting MMP-2 and MMP-9 (5,13,14). Furthermore, dox-
ycycline inhibits cancer progression through a number 
of signalling pathways (5,8). Among others, it directly 
inhibits protease-activated receptor 1 (PAR-1) signalling 
and downregulates VEGF signalling (10,15,16). Doxycy-
cline also inhibits tumour cell adhesion, migration and 
invasion by inhibiting the expression and phosphoryla-
tion of adhesion molecules such as focal adhesion kinase 
(FAK) in leukaemia cells and melanoma cells (11,12). It 
also reduces angiogenesis and cancer cell migration by 
inhibiting the expression of interleukin 8 (IL-8) (17). By 
inhibiting doxycycline signalling pathways, it inhibits 
tumour cell invasion, epithelial-mesenchymal transition 
of tumour cells and metastasis formation (5,8,14).
Doxycycline induces apoptosis and inhibits cell pro-
liferation (8,17). Inhibition of apoptosis involves both 
the mitochondria-dependent and caspase-dependent 
pathways (8,18). Doxycycline has been shown to induce 
cell cycle arrest in the G2/M phase in prostate cancer 
cells (19). According to the results in different types of 
cancer, doxycycline promotes apoptosis and inhibits cell 
proliferation by inhibiting various molecular targets that 
218
CYTOLOGY, ONCOLOGY, CANCEROLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3188
are importantly involved in these processes (8).
Doxycycline is as an antitumour agent of particular 
interest for its action against cancer stem cells (8). In cer-
vical cancer, doxycycline has been shown to reduce the 
expression of cancer stem cell markers such as SOX-2, 
OCT-4, NANOG and NOTCH (20).
The anti-tumour activity of doxycycline has also been 
evaluated in clinical studies. However, in a phase 2 clini-
cal study in patients with breast cancer and bone metas-
tases, doxycycline did not show significant effects (21), 
while in a small pilot clinical study involving 15 patients, 
it showed a positive effect in almost 90% of persons with 
early forms of breast cancer, who received doxycycline 
14 days prior to surgery. The expression of cancer stem 
cell markers (22) decreased in tumour samples following 
doxycycline treatment, showing promising possibilities 
for its use in cancer therapy. However, this needs to be 
further evaluated in a larger clinical study.
Doxycycline is also effective in combination with oth-
er known chemotherapeutic agents, making it a promis-
ing active substance already in clinical use that could be 
used in oncology to treat cancer alone or in combination 
with conventional therapy to treat cancer and prevent its 
recurrence (8).
3 Chloroquine
Chloroquine (7-Chloro-4-(4-Diethylamino-1-Meth-
ylbutylamino)-Quinoline) (Figure 1) and its analogue 
hydroxychloroquine are used for the prevention and 
treatment of malaria (23-25). Chloroquine is an old 
drug, synthesised as early as 1934. It was used as the 
compound of choice in the treatment of malaria until 
the emergence of resistant strains (24). Later, its use 
was extended to the treatment of rheumatoid arthritis 
and lupus erythematosus (23,25). Chloroquine’s anti-
tumour activity was first observed in the 1970s when, 
during a study of its effect against malaria, the incidence 
of Burkitt’s lymphoma was reduced in a group receiving 
chloroquine. However, it was only later that the latter 
attracted more interest (26). To date, the antitumour ac-
tivity of chloroquine and hydroxychloroquine has been 
demonstrated in several in vitro and in vivo studies in 
mouse models with different types of cancer, including 
breast, liver and colon cancer, glioblastoma and melano-
ma. It reduced tumour progression and slowed tumour 
growth (25). In a mouse model of liver cancer, a reduc-
tion in the number and size of metastases in the lungs 
was also shown (27).
As antitumour agents, chloroquine and hydroxy-
chloroquine can be used individually or as supportive 
therapies to improve the effect of chemotherapy and ra-
diotherapy in different types of cancer. They are current-
ly involved in several clinical studies (23-25). As of Feb-
ruary 2021, 21 clinical trials for chloroquine in cancer 
and 89 clinical trials for hydroxychloroquine have been 
registered on clinicaltrials.gov, which are at various stag-
es, many of the trials still ongoing, with results available 
for only some of the trials (28). In clinical trials, patients 
mainly received chloroquine and hydroxychloroquine 
in combination with other antitumour agents (25,28). 
Results to date show that for some types of cancer, pa-
tients receiving chloroquine or hydroxychloroquine as 
supportive therapy have prolonged survival compared 
to the control group (25,29). A significant improvement 
in survival of patients receiving chloroquine in addition 
to radiotherapy and chemotherapy was already observed 
in one of the first clinical trials, which started in May 
1998, in 18 patients with glioblastoma (30). A similar 
improvement in survival rate of glioblastoma patients 
who received chloroquine in addition to conventional 
chemotherapy and radiation, when compared to those 
who received placebo, was also observed in a clinical 
study conducted in October 2000. The study included 
15 patients in each group (31). The retrospective study 
pooled all data obtained over a 5-year period from 41 
glioblastoma patients who received chloroquine as ad-
juvant therapy and were not included in the aforemen-
tioned study (31). In two independent phase 2 clinical 
studies (32) and a prospective study, in one group (33) of 
patients with brain metastases, chloroquine was shown 
to increase the sensitivity of cells to radiotherapy in pa-
tients with brain metastases. It prolongs both the metas-
tasis progression-free survival and overall survival, sug-
gesting the possibility of its use as a supportive treatment 
for radiation therapy (25). Furthermore, several clinical 
studies of different types of cancer have shown that the 
addition of hydroxychloroquine to conventional therapy 
improves response to therapy by inhibiting autophagy 
(25).
Due to their long-standing use in clinical practice, 
the side-effects of chloroquine and hydroxychloroquine 
are well known. Serious side effects rarely occur after 
their short-term use. However, adverse toxic effects, in 
particular nephrotoxicity, may occur after prolonged 
use at higher doses and when taken concomitantly with 
other antitumour agents (23,25,34). The presence of an 
additional hydroxy group in hydroxychloroquine sig-
nificantly reduces toxicity and affects the pharmacoki-
netic properties of the compound, while the differences 
in antitumour activity of chloroquine and hydroxychlo-
roquine are not fully known (25,35).
219
REVIEW ARTICLE
Possible use of antimicrobial agents in cancer therapy
Chloroquine and hydroxychloroquine are highly sol-
uble and are rapidly and well absorbed after oral admin-
istration (24,25). Chloroquine is a weak base; at physio-
logical pH 7.4, it is found in an unprotonated or partially 
protonated form, which allows it to pass well through 
cell membranes and enter acidic cell compartments. As 
a weak base, chloroquine is protonated and retained in 
lysosomes after entering the acidic environment of ly-
sosomes. This also results in an increase in the pH of 
the lysosomes, which reduces their function, as most of 
the proteins in lysosomes have their optimum activity at 
acidic pH (23-25).
The retention of chloroquine inside cells leads to 
inhibition of autophagy, one of the key mechanisms 
of its antitumour activity (23-25,34). Chloroquine and 
hydroxychloroquine inhibit autophagy by inhibiting 
autophagosome fusion and degradation within lyso-
somes due to an increase in pH (25,36). It also affects 
autophagy via the PI3K/Akt/mTOR signalling path-
way, where it has a synergistic effect with inhibitors of 
the AMPK signalling pathway and the activated Janus 
kinase-2 (JAK2)/STAT3 signalling pathway (23,25,37). 
In addition to early autophagy, chloroquine also inhibits 
late stages of autophagy and can induce cell death even 
under conditions where inhibitors of early stages of au-
tophagy cannot (23).
Several independent studies have also shown an 
antitumour activity of chloroquine, independent of in-
hibition of autophagy (24). One of the mechanisms is 
modulation of cellular metabolism, affecting amino ac-
id metabolism, glucose metabolism and mitochondrial 
metabolism (23,24). Chloroquine and hydroxychloro-
quine also affect apoptosis and cell cycle arrest by regu-
lating molecules importantly involved in these processes 
(25). Moreover, chloroquine can also induce cell death 
via lysosome-mediated apoptosis by acting on cathep-
sins (24). Chloroquine is also involved in the regula-
tion of cellular stress and modulators of inflammation 
by regulating proteostasis via the ubiquitin/proteasome 
system (23). Next, it affects cell proliferation and surviv-
al by acting on glutamate dehydrogenase activity (25). 
Chloroquine also exerts antitumour effects by normalis-
ing tumour vasculature by reducing vascular density and 
increasing tight junction formation (25,38).
Several molecular targets of the antitumour activity 
of chloroquine and hydroxychloroquine are known, in-
cluding the NF-κB transcription factor, p53 tumour sup-
pressor factor and CXCL12/CXCR4 signalling pathways 
(24,25). By inhibiting the CXCL12/CXCR4 signalling 
pathway, chloroquine reduces phosphorylation of the 
signalling molecules ERK and STAT3 (25) and thereby 
also acts on cancer stem cells (23). Additionally, in a 
bile duct cancer cell line, chloroquine inhibits tumour 
cell metastasis by affecting hypoxia-inducible factor 1α 
(HIF-1α), VEGF and the epithelial-mesenchymal tran-
sition process (25).
By inhibiting autophagy, chloroquine can bypass the 
limitations of existing chemotherapy, as autophagy is, 
among other things, one of the key processes for sur-
vival and resistance to existing chemotherapy in cancer 
stem cells and facilitates the epithelial-mesenchymal 
transition of tumour cells. This is one of the process-
es by which cells can acquire the properties of cancer 
stem cells (23,25). In cancer stem cells, chloroquine at 
a concentration of 20 μM was shown to affect stemness 
by specific action on autophagy (23,39). It was further 
shown that in cancer stem cells, low micromolar con-
centrations of chloroquine affect JAK2/STAT3, Hedge-
hog and CXC4 signalling pathways, DNA methyl trans-
ferase expression and expression of stemness markers 
(23,25,40). Chloroquine has also been shown in several 
studies to be involved in regulating the immune system 
and the inflammatory response through various mech-
anisms (23,24).
When in concomitant use, chloroquine improves the 
effect of chemotherapy and radiotherapy. By inhibiting 
autophagy, it increases the sensitivity of tumour cells to 
radiotherapy, which has been shown in various types of 
cancer including breast cancer, lung cancer, and glioma. 
A meta-analysis of studies has shown that addition of 
chloroquine to chemotherapy or radiotherapy increases 
patient survival and prolongs disease-free survival (23). 
Chloroquine and hydroxychloroquine have a synergis-
tic effect with BET inhibitors already at low concentra-
tions and induce cancer stem cell apoptosis in pancreatic 
cancer and acute myeloid leukaemia (23,41,42). Chlo-
roquine also increases the efficiency and killing of tu-
mour cells in combination with cell cycle inhibitors and 
leads to apoptosis together with proteasome inhibitors, 
as shown in a mouse model of liver cancer (23,43). Fur-
thermore, inhibitors of the AMPK signalling pathway 
and the STAT3 signalling pathway have also been shown 
to have a synergistic effect on cell death (23).
Despite promising results from pre-clinical studies, 
chloroquine significant antitumour effect in clinical 
studies was not yet shown. Key reasons for this could 
be that chloroquine is less able to enter tumour cells due 
to the acidic extracellular environment of tumours, and 
that chloroquine has an important role in regulating 
the immune response by inhibiting autophagy in lyso-
somes (23,25). In addition, the different dependence of 
tumours on autophagy results in different responses to 
220
CYTOLOGY, ONCOLOGY, CANCEROLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3188
chloroquine therapy in different types of tumours (25). 
At the same time, it was shown that high concentrations 
of chloroquine are required to achieve a therapeutic ef-
fect on autophagy inhibition in leukaemia, which lim-
its its use. Nevertheless, chloroquine has been shown to 
affect metastatic tumour cells that are not in an acidic 
tumour environment and are particularly sensitive to 
lysosomal inhibition. This reduces the possibility of me-
tastasis formation (23). In triple-negative breast cancer, 
chloroquine successfully kills cancer stem cells and re-
duces the ability of tumours to metastasise in both in 
vitro and in vivo mouse models (44). Chloroquine has 
been shown to act primarily on tumour cells with can-
cer stem cell properties. Therefore, its use is particularly 
appropriate in the context of concomitant use of agents 
targeting differentiated tumour cells (23,25).
4 Nitroxoline
Nitroxoline (5-nitro-8-hydroxyquinoline) (Figure 1) 
is a well-known antimicrobial agent used in the treat-
ment of urinary tract infections. Nitroxoline’s antimicro-
bial activity was discovered in the 1950s and it was fast 
used in clinical practice (45). Despite it being used for 
a long time, bacteria have still not developed resistance 
to nitroxoline (45,46). Nitroxoline is effective against 
most Gram-positive and Gram-negative pathogenic uri-
nary tract bacteria, mycoplasmas (M. hominis, Ureaplas-
ma urealyticum) and human pathogens Candida spp. 
(45). In addition, nitroxoline showed efficacy against 
most other Gram-negative bacteria and was also effec-
tive against Gram-positive bacteria, suggesting poten-
tial use as an antimicrobial agent for other indications 
in addition to the treatment of urinary tract infections 
(47). Nitroxoline exerts its antimicrobial activity mainly 
by chelation of various bivalent ions (45,48,49). In this 
way, it stabilises lipopolysaccharides on the bacterial 
surface, which increases the hydrophobicity of the bac-
terial surface and reduces their adhesion (45,48,49). By 
this nitroxoline also inhibits RNA synthesis during yeast 
cell division (45,50), while at lower concentrations, it in-
hibits the formation of bacterial biofilms (45,46,51). As 
an established antimicrobial agent, nitroxoline has well-
known pharmacokinetic and pharmacodynamic prop-
erties (45,52-54). Nitroxoline is completely and rapidly 
absorbed in the urinary tract after oral administration 
(52,54), and only minor and tolerable side effects have 
been observed, suggesting that nitroxoline administra-
tion is safe (45).
Antitumour activity of nitroxoline has been demon-
strated by several studies to date. Its antitumour activity 
was first identified in the search for methionine amino-
peptidase type 2 (MetAP2) inhibitors, which could be 
used as new anti-angiogenic agents (55). Nitroxoline has 
been shown as potent inhibitor of MetAP2 in vitro, in-
hibiting endothelial cell proliferation and angiogenesis 
both in vitro and in vivo. In addition, nitroxoline induces 
senescence and inhibits HUVEC endothelial cell prolif-
eration by simultaneously inhibiting MetAP2 and Sir-
tuin 1 (Sirt1). By Sirt1 inhibition, nitroxoline increases 
the amount of acetylated tumour suppressor p53, result-
ing in reduced angiogenesis. At the same time, nitrox-
oline significantly reduced the size of breast tumours 
in a mouse model and inhibited the growth of bladder 
cancer in an orthotopic mouse model (55). Nitroxoline 
was later shown to reduce the expression of proteins as-
sociated with the epithelial-mesenchymal transition of 
tumour cells and the amount of myeloid-derived sup-
pressor cells (MDSCs) in peripheral blood in bladder 
cancer (56). Furthermore, nitroxoline was identified in 
a second, independent study by high throughput com-
pound library search and biochemical evaluation of the 
best hits as a potent, reversible and non-covalent in-
hibitor of the lysosomal cysteine peptidase cathepsin B 
(57), which plays an important role in the initiation and 
progression of cancer, its invasion and metastasis, and is 
known to be a promising target for the development of 
novel antitumour agents (58-60). The crystal structure of 
the nitroxoline-cathepsin B complex shows that nitroxo-
line binds to the active site cleft of cathepsin B and there-
by selectively inhibits its endopeptidase activity at low 
micromolar concentrations (57). In various functional 
assays in cell lines and murine tumour models, nitrox-
oline significantly reduced extracellular matrix degra-
dation, tumour cell invasion, metastasis and endothelial 
tube formation in an in vitro model of angiogenesis (58).
Nitroxoline acts as an antitumour agent and inhibits 
tumour cell migration by reducing the expression of the 
tumour transcription factor FOXM1, an important reg-
ulator of cancer progression. In the same study, nitroxo-
line was also shown to reduce the expression of MMP-2 
and MMP-9, which are important players in tumour mi-
gration and invasion and whose expression is controlled 
by, among others, FOXM1. In addition to inhibition of 
MMP-2 and MMP-9 by inhibiting FOXM1, molecular 
interaction studies suggest that nitroxoline also direct-
ly interacts with the catalytic domains of MMP-2 and 
MMP-9 (61).
Furthermore, several studies have shown that nitrox-
oline inhibits tumour cell proliferation. This has been 
shown in cells from a variety of cancer types, includ-
ing malignant glioma, multiple myeloma, glioblastoma, 
221
REVIEW ARTICLE
Possible use of antimicrobial agents in cancer therapy
leukaemia, prostate cancer, bile duct cancer (cholangio-
carcinoma), pancreatic cancer, small cell lung cancer 
and bladder cancer (56,62). The antitumour activity of 
nitroxoline has been shown to be dose-dependent, as 
well as dependent on the period of administration, with 
lower doses leading mainly to cell cycle arrest in the G0/
G1 phase, while higher doses promote apoptosis. Ni-
troxoline promotes apoptosis and cell cycle arrest in the 
G0/G1 phase by acting on various molecular targets that 
are importantly involved in these processes (62). The an-
titumour activity of nitroxoline on pancreatic cell lines 
was further improved when nitroxoline was co-adminis-
tered together with nelfinavir, a competitive inhibitor of 
HIV aspartate peptidase, and other antiviral agents for 
the treatment of HIV infection (63). However, according 
to data available at clinicaltrials.gov, the antitumour ac-
tivity of nitroxoline has not yet been evaluated in clinical 
studies (28).
To further improve the antitumour activity of nitrox-
oline, several new derivatives with different structural 
modifications have been prepared by structure-based 
chemical synthesis, based on the structure of nitroxoline 
(62). By modifying various substituents, a large number 
of nitroxoline derivatives was prepared with the aim to 
improve cathepsin B inhibition (57,64-66), its anti-an-
giogenic action by inhibiting MetAP2 and SIRT1 (67) 
and inhibition of BET proteins, which bind competitive-
ly to BRD-BD1 and are involved in maintaining chro-
matin stability, thereby controlling cell cycle progression 
(62,68,69). The preparation of new derivatives also re-
vealed the structural features needed to improve activity 
on a particular target (62). Additionally, the antitumour 
activity of nitroxoline and the inhibition of cathepsin 
B were also improved by the preparation of organoru-
thenium complexes with nitroxoline and its derivatives 
(70). The antitumour activity of nitroxoline, and in par-
ticular its uptake into lysosomes, was also improved by 
its incorporation into nanoparticles composed of bovine 
serum albumin, copper ions and nitroxoline (BSA/Cu/
NQ) (71).
Nitroxoline is a promising, known active substance 
that could be used to treat various types of cancer. Ni-
troxoline is also an interesting compound for the devel-
opment of new derivatives with improved antitumour 
activity and selectivity for specific targets.
5 Fluoroquinolones
Fluoroquinolones (Figure 1) are the largest group of 
antimicrobial agents. Fluoroquinolones inhibit bacterial 
DNA duplication and transcription, either by inhibiting 
bacterial DNA gyrase or topoisomerase II (4). Nalidixic 
acid was discovered as the first quinolone with antimi-
crobial activity in the 1960s. The discovery was followed 
by further development and optimization of quinolones. 
They can be divided into four generations based on their 
pharmacokinetic and antimicrobial properties. Initially, 
quinolones were only effective against Gram-negative 
bacteria, but as their structure has been optimized, their 
spectrum of activity has broadened. Fourth-generation 
fluoroquinolones are effective against Gram-negative 
and Gram-positive bacteria, anaerobes Pseudomonas sp. 
and atypical bacteria (72). In addition to their antibac-
terial activity, some fluoroquinolones have been shown 
to be effective in combination with chloroquine in the 
treatment of malaria. They have also been shown to 
work against some other parasites, such as Trypanosoma 
brucei and Toxoplasma gondii (4). In addition to their 
direct effect on bacterial growth, fluoroquinolones con-
tribute to antimicrobial activity by their immunomodu-
latory properties. Fluoroquinolones, especially those of 
the newer generation such as ciprofloxacin, norfloxacin 
and ofloxacin, inhibit cytokine synthesis even at low 
concentrations. They have also displayed an anti-inflam-
matory response and a protective role against lipopoly-
saccharide-induced liver injury (4,73).
Several independent studies have demonstrated an-
titumour activity of fluoroquinolones, with multiple 
different mechanisms of antitumour action. Different 
fluoroquinolones have been shown to stop tumour pro-
gression with cell cycle arrest, mainly in the S/G2 phase, 
and by inducing tumour cell apoptosis (4). Ciprofloxacin 
has been shown as the most potent inducer of apoptosis 
and has induced apoptosis in cells from a variety of can-
cers, including prostate, bladder, pancreatic, colorectal 
cancer and others (4,74,75). In contrast, a more recent 
study of ciprofloxacin showed the opposite effect on can-
cer progression, showing it promotes the formation of 
cells with a cancer stem cell phenotype in human small 
cell lung cancer cells. Here, ciprofloxacin was shown to 
increase the expression of stemness markers and pro-
teins required for cell self-renewal (76). Among fluoro-
quinolones, only gemifloxacin has been shown to inhibit 
tumour cell metastasis by inhibiting migration and inva-
sion through inhibition of TNFα-stimulated NFκB acti-
vation (77,78). Additionally, sparfloxacin was shown to 
have an impact on invasion and migration of colon can-
cer tumour cells (79). Enoxacin, due to its unique struc-
ture, shows antitumour effects by inhibiting the produc-
tion of microRNA (miRNA), because it is oncogenic and 
contributes to tumour progression (4,80,81). Several in-
dependent studies showed fluoroquinolones, including 
222
CYTOLOGY, ONCOLOGY, CANCEROLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3188
References
1. World Health Organization. Cancer. Geneva: WHO; 2020 [cited 2021 Jan 
17]. Available from: https://www.who.int/news-room/fact-sheets/detail/
cancer.
2. Gupta SC, Sung B, Prasad S, Webb LJ, Aggarwal BB. Cancer drug discovery 
by repurposing: teaching new tricks to old dogs. Trends Pharmacol Sci. 
2013;34(9):508-17. DOI: 10.1016/j.tips.2013.06.005 PMID: 23928289 
3. Shim JS, Liu JO. Recent advances in drug repositioning for the discovery 
of new anticancer drugs. Int J Biol Sci. 2014;10(7):654-63. DOI: 10.7150/
ijbs.9224 PMID: 25013375 
4. Yadav V, Talwar P. Repositioning of fluoroquinolones from antibiotic to 
anti-cancer agents: an underestimated truth. Biomed Pharmacother. 
2019;111(111):934-46. DOI: 10.1016/j.biopha.2018.12.119 PMID: 30841473 
ciprofloxacin, fleroxacin, moxifloxacin and enoxacin, 
improve the antitumour effects of chemotherapeutic 
agents when co-administered with existing chemother-
apeutic agents used in clinical practice (4). Several clini-
cal trials are also currently underway to test the anti-tu-
mour activity of fluoroquinolones in combination with 
established therapies in different types of cancer, such as 
bladder cancer and acute myeloid leukaemia. The final 
results have yet to be published (28).
The antitumour activity of fluoroquinolones can be 
further enhanced by the formation of complexes with 
metal ions. The latter was first demonstrated for com-
plexes of norfloxacin with copper, which showed im-
proved antitumour activity against leukaemia cells (82). 
This led to the preparation of metal complexes with oth-
er fluoroquinolones, further improving the anti-tumour 
activity against different cancer cell lines compared to 
the initial compound. The gold-ruthenium complexes 
proved to be particularly effective in this regard, with an-
titumour activity mainly against metastatic tumour cells, 
but without toxic effects on normal, healthy cells. The 
antitumour activity of ruthenium complexes is thought 
to be due to action on multiple targets, while the antitu-
mour activity of gold complexes is thought to be mainly 
due to alteration of mitochondrial function and inhibi-
tion of protein synthesis. Additionally, the introduction 
of nitrogen adducts into metal complexes with fluoro-
quinolones inhibits their elimination from cells, thus al-
lowing for increased retention of active compounds in 
tumours (4).
Moreover, research on fluoroquinolones is also fo-
cused on finding structural modifications to the mole-
cules of existing fluoroquinolones that could further im-
prove their antitumour activity. The presence of major 
functional groups at the C-7 site has been shown to be 
beneficial for enhancing antitumour activity (4,83).
The large group of fluoroquinolones represents a 
promising group of antimicrobial drugs whose use could 
be extended to the treatment of cancer due to their an-
titumour activity. At the same time, structural modifi-
cations of existing fluoroquinolones could lead to the 
development of new molecules with improved antitu-
mour activity.
6 Conclusion
Successful cancer treatment still faces many chal-
lenges. One of the important steps is to develop new 
drugs to avoid the limitations of current therapeutic 
approaches, such as resistance to therapy and the ma-
ny side effects, associated with existing chemotherapeu-
tics. However, the high costs associated with bringing a 
new drug to market have recently led to an increasing 
focus on finding new indications for existing drugs. It 
was shown that some antimicrobial drugs that have long 
been used successfully in clinical practice have antitu-
mour activity. Among the latter, antitumour activity 
was also demonstrated for doxycycline, chloroquine, 
nitroxoline and fluoroquinolones that are discussed in 
detail in this review article. Their antitumour activity 
has been demonstrated in a number of tumour models 
and on different cancer types, and different mechanisms 
of antitumour action have been described. Known anti-
microbial agents with antitumour activity also represent 
interesting compounds for the development of new de-
rivatives with improved antitumour activity. Despite the 
numerous studies confirming the antitumour activity of 
anti-microbial agents, further clinical studies are needed 
to confirm their efficacy and allow their successful in-
troduction into clinical practice. Attention should also 
be paid to possible interactions with existing therapies, 
in particular immune therapies and antibiotic therapy, 
which cancer patients often receive because of infections 
that result from their weakened immune system. Drug 
repurposing is therefore a promising approach that will 
allow faster and more efficient development of new an-
ti-tumour therapies and improve the success of cancer 
treatment.
Conflict of interest
None declared.
223
REVIEW ARTICLE
Possible use of antimicrobial agents in cancer therapy
5. Sleire L, Førde HE, Netland IA, Leiss L, Skeie BS, Enger PØ. Drug 
repurposing in cancer. Pharmacol Res. 2017;124:74-91. DOI: 10.1016/j.
phrs.2017.07.013 PMID: 28712971 
6. Hernandez JJ, Pryszlak M, Smith L, Yanchus C, Kurji N, Shahani VM, et 
al. Giving drugs a second chance: overcoming regulatory and financial 
hurdles in repurposing approved drugs as cancer therapeutics. Front 
Oncol. 2017;7(NOV):273. DOI: 10.3389/fonc.2017.00273 PMID: 29184849 
7. Nowak-Sliwinska P, Scapozza L, Ruiz i Altaba A. Drug repurposing in 
oncology: Compounds, pathways, phenotypes and computational 
approaches for colorectal cancer. Biochim Biophys Acta Rev Cancer. 
2019;1871(2):434-54. DOI: 10.1016/j.bbcan.2019.04.005 PMID: 31034926 
8. Antoszczak M, Markowska A, Markowska J, Huczyński A. Old wine in new 
bottles: drug repurposing in oncology. Eur J Pharmacol. 2019;2020:866. 
PMID: 31730760 
9. van den Bogert C, Dontje BH, Kroon AM. The antitumour effect of 
doxycycline on a T-cell leukaemia in the rat. Leuk Res. 1985;9(5):617-23. 
DOI: 10.1016/0145-2126(85)90142-0 PMID: 3874329 
10. Wang SQ, Zhao BX, Liu Y, Wang YT, Liang QY, Cai Y, et al. New application 
of an old drug: antitumor activity and mechanisms of doxycycline in 
small cell lung cancer. Int J Oncol. 2016;48(4):1353-60. DOI: 10.3892/
ijo.2016.3375 PMID: 26846275 
11. Wang C, Xiang R, Zhang X, Chen Y. Doxycycline inhibits leukemic 
cell migration via inhibition of matrix metalloproteinases and 
phosphorylation of focal adhesion kinase. Mol Med Rep. 2015;12(3):3374-
80. DOI: 10.3892/mmr.2015.3833 PMID: 26004127 
12. Sun T, Zhao N, Ni CS, Zhao XL, Zhang WZ, Su X, et al. Doxycycline 
inhibits the adhesion and migration of melanoma cells by inhibiting 
the expression and phosphorylation of focal adhesion kinase (FAK). 
Cancer Lett. 2009;285(2):141-50. DOI: 10.1016/j.canlet.2009.05.004 PMID: 
19482420 
13. Shen LC, Chen YK, Lin LM, Shaw SY. Anti-invasion and anti-tumor 
growth effect of doxycycline treatment for human oral squamous-cell 
carcinoma—in vitro and in vivo studies. Oral Oncol. 2010;46(3):178-84. 
DOI: 10.1016/j.oraloncology.2009.11.013 PMID: 20036604 
14. Qin Y, Zhang Q, Lee S, Zhong WL, Liu YR, Liu HJ, et al. Doxycycline reverses 
epithelial-to-mesenchymal transition and suppresses the proliferation 
and metastasis of lung cancer cells. Oncotarget. 2015;6(38):40667-79. 
DOI: 10.18632/oncotarget.5842 PMID: 26512779 
15. Zhong W, Chen S, Zhang Q, Xiao T, Qin Y, Gu J, et al. Doxycycline 
directly targets PAR1 to suppress tumor progression. Oncotarget. 
2017;8(10):16829-42. DOI: 10.18632/oncotarget.15166 PMID: 28187433 
16. Zhong W, Chen S, Qin Y, Zhang H, Wang H, Meng J, et al. Doxycycline 
inhibits breast cancer EMT and metastasis through PAR-1/NF-κB/miR-17/
E-cadherin pathway. Oncotarget. 2017;8(62):104855-66. DOI: 10.18632/
oncotarget.20418 PMID: 29285218 
17. Son K, Fujioka S, Iida T, Furukawa K, Fujita T, Yamada H, et al. Doxycycline 
induces apoptosis in PANC-1 pancreatic cancer cells. Anticancer Res. 
2009;29(10):3995-4003. PMID: 19846942 
18. Song H, Fares M, Maguire KR, Sidén Å, Potácová Z. Cytotoxic effects of 
tetracycline analogues (doxycycline, minocycline and COL-3) in acute 
myeloid leukemia HL-60 cells. PLoS One. 2014;9(12):e114457. DOI: 
10.1371/journal.pone.0114457 PMID: 25502932 
19. Zhu C, Yan X, Yu A, Wang Y. Doxycycline synergizes with doxorubicin to 
inhibit the proliferation of castration-resistant prostate cancer cells. Acta 
Biochim Biophys Sin (Shanghai). 2017;49(11):999-1007. DOI: 10.1093/
abbs/gmx097 PMID: 28985240 
20. Yang B, Lu Y, Zhang A, Zhou A, Zhang L, Zhang L, et al. Doxycycline 
Induces Apoptosis and Inhibits Proliferation and Invasion of Human 
Cervical Carcinoma Stem Cells. PLoS One. 2015. ;10(6)DOI: 10.1371/
journal.pone.0129138 PMID: 226111245 
21. Addison CL, Simos D, Wang Z, Pond G, Smith S, Robertson S, et al. A 
phase 2 trial exploring the clinical and correlative effects of combining 
doxycycline with bone-targeted therapy in patients with metastatic breast 
cancer. J Bone Oncol. 2016;5(4):173-9. DOI: 10.1016/j.jbo.2016.06.003 
PMID: 28008379 
22. Scatena C, Roncella M, Di Paolo A, Aretini P, Menicagli M, Fanelli G, et al. 
Doxycycline, an Inhibitor of Mitochondrial Biogenesis, Effectively Reduces 
Cancer Stem Cells (CSCs) in Early Breast Cancer Patients: A Clinical Pilot 
Study. Front Oncol. 2018;8:452. DOI: 10.3389/fonc.2018.00452 PMID: 
30364293 
23. Varisli L, Cen O, Vlahopoulos S. Dissecting pharmacological effects 
of chloroquine in cancer treatment: interference with inflammatory 
signaling pathways. Immunology. 2020;159(3):257-78. DOI: 10.1111/
imm.13160 PMID: 31782148 
24. Weyerhäuser P, Kantelhardt SR, Kim EL. Re-purposing chloroquine for 
glioblastoma: potential merits and confounding variables. Front Oncol. 
2018;8(AUG):335. DOI: 10.3389/fonc.2018.00335 PMID: 30211116 
25. Verbaanderd C, Maes H, Schaaf MB, Sukhatme VP, Pantziarka P, 
Sukhatme V, et al. Repurposing Drugs in Oncology (ReDO)-chloroquine 
and hydroxychloroquine as anti-cancer agents. Ecancermedicalscience. 
2017;11:781. DOI: 10.3332/ecancer.2017.781 PMID: 29225688 
26. Geser A, Brubaker G, Draper CC. Effect of a malaria suppression program 
on the incidence of African Burkitt’s lymphoma. Am J Epidemiol. 
1989;129(4):740-52. DOI: 10.1093/oxfordjournals.aje.a115189 PMID: 
2923122 
27. Jiang PD, Zhao YL, Deng XQ, Mao YQ, Shi W, Tang QQ, et al. Antitumor 
and antimetastatic activities of chloroquine diphosphate in a murine 
model of breast cancer. Biomed Pharmacother. 2010;64(9):609-14. DOI: 
10.1016/j.biopha.2010.06.004 PMID: 20888174 
28. National Library of Medicine. ClinicalTrials.gov. Bethesda: US NLM; 2021 
[cited 2021 Feb 13]. Available from: https://clinicaltrials.gov.
29. Xu R, Ji Z, Xu C, Zhu J. The clinical value of using chloroquine or 
hydroxychloroquine as autophagy inhibitors in the treatment of 
cancers: A systematic review and meta-analysis. Medicine (Baltimore). 
2018;97(46):e12912. DOI: 10.1097/MD.0000000000012912 PMID: 
30431566 
30. Briceño E, Reyes S, Sotelo J. Therapy of glioblastoma multiforme 
improved by the antimutagenic chloroquine. Neurosurg Focus. 
2003;14(2):e3. DOI: 10.3171/foc.2003.14.2.4 PMID: 15727424 
31. Sotelo J, Briceño E, López-González MA. Adding chloroquine to 
conventional treatment for glioblastoma multiforme: a randomized, 
double-blind, placebo-controlled trial. Ann Intern Med. 2006;144(5):337-
43. DOI: 10.7326/0003-4819-144-5-200603070-00008 PMID: 16520474 
32. Rojas-Puentes LL, Gonzalez-Pinedo M, Crismatt A, Ortega-Gomez 
A, Gamboa-Vignolle C, Nuñez-Gomez R, et al. Phase II randomized, 
double-blind, placebo-controlled study of whole-brain irradiation 
with concomitant chloroquine for brain metastases. Radiat Oncol. 
2013;8(1):209. DOI: 10.1186/1748-717X-8-209 PMID: 24010771 
33. Eldredge HB, Denittis A, Duhadaway JB, Chernick M, Metz R, Prendergast 
GC. Concurrent whole brain radiotherapy and short-course chloroquine 
in patients with brain metastases: a pilot trial. J Radiat Oncol. 
2013;2(3):315-21. DOI: 10.1007/s13566-013-0111-x PMID: 24187608 
34. Kimura T, Takabatake Y, Takahashi A, Isaka Y. Chloroquine in cancer 
therapy: a double-edged sword of autophagy. Cancer Res. 2013;73(1):3-
7. DOI: 10.1158/0008-5472.CAN-12-2464 PMID: 23288916 
35. Ben-Zvi I, Kivity S, Langevitz P, Shoenfeld Y. Hydroxychloroquine: from 
malaria to autoimmunity. Clin Rev Allergy Immunol. 2012;42(2):145-53. 
DOI: 10.1007/s12016-010-8243-x PMID: 21221847 
36. Townsend KN, Hughson LR, Schlie K, Poon VI, Westerback A, Lum JJ. 
Autophagy inhibition in cancer therapy: metabolic considerations 
for antitumor immunity. 2012;249(1):176-94. DOI: 10.1111/j.1600-
065X.2012.01141.x PMID: 22889222 
37. Loehberg CR, Strissel PL, Dittrich R, Strick R, Dittmer J, Dittmer A, et al. 
Akt and p53 are potential mediators of reduced mammary tumor growth 
by cloroquine and the mTOR inhibitor RAD001. Biochem Pharmacol. 
2012;83(4):480-8. DOI: 10.1016/j.bcp.2011.11.022 PMID: 22142888 
38. Maes H, Kuchnio A, Peric A, Moens S, Nys K, De Bock K, et al. Tumor vessel 
normalization by chloroquine independent of autophagy. Cancer Cell. 
2014;26(2):190-206. DOI: 10.1016/j.ccr.2014.06.025 PMID: 25117709 
224
CYTOLOGY, ONCOLOGY, CANCEROLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3188
39. Antonelli M, Strappazzon F, Arisi I, Brandi R, D’Onofrio M, Sambucci M, 
et al. ATM kinase sustains breast cancer stem-like cells by promoting 
ATG4C expression and autophagy. Oncotarget. 2017;8(13):21692-709. 
DOI: 10.18632/oncotarget.15537 PMID: 28423511 
40. Choi DS, Blanco E, Kim YS, Rodriguez AA, Zhao H, Huang TH, et al. 
Chloroquine eliminates cancer stem cells through deregulation of Jak2 
and DNMT1. Stem Cells. 2014;32(9):2309-23. DOI: 10.1002/stem.1746 
PMID: 24809620 
41. Sakamaki J, Wilkinson S, Hahn M, Tasdemir N, O’Prey J, Clark W, et al. 
Brodomain protein BRD4 is a transciptional repressor of autophagy 
and lysosomal function. Mol Cell. 2017;66(4):P517-32. DOI: 10.1016/j.
molcel.2017.04.027 PMID: 28525743
42. Jang JE, Eom JI, Jeung HK, Cheong JW, Lee JY, Kim JS, et al. AMPK-ULK1-
Mediated Autophagy Confers Resistance to BET Inhibitor JQ1 in Acute 
Myeloid Leukemia Stem Cells. Clin Cancer Res. 2017;23(11):2781-94. DOI: 
10.1158/1078-0432.CCR-16-1903 PMID: 27864418 
43. Hui B, Shi YH, Ding ZB, Zhou J, Gu CY, Peng YF, et al. Proteasome 
inhibitor interacts synergistically with autophagy inhibitor to suppress 
proliferation and induce apoptosis in hepatocellular carcinoma. Cancer. 
2012;118(22):5560-71. DOI: 10.1002/cncr.27586 PMID: 22517429 
44. Liang DH, Choi DS, Ensor JE, Kaipparettu BA, Bass BL, Chang JC. The 
autophagy inhibitor chloroquine targets cancer stem cells in triple 
negative breast cancer by inducing mitochondrial damage and impairing 
DNA break repair. Cancer Lett. 2016;376(2):249-58. DOI: 10.1016/j.
canlet.2016.04.002 PMID: 27060208 
45. Naber KG, Niggemann H, Stein G, Stein G. Review of the literature and 
individual patients’ data meta-analysis on efficacy and tolerance of 
nitroxoline in the treatment of uncomplicated urinary tract infections. 
BMC Infect Dis. 2014;14(1):628. DOI: 10.1186/s12879-014-0628-7 PMID: 
25427651 
46. Kresken M, Körber-Irrgang B. In vitro activity of nitroxoline against 
Escherichia coli urine isolates from outpatient departments in Germany. 
Antimicrob Agents Chemother. 2014;58(11):7019-20. DOI: 10.1128/
AAC.03946-14 PMID: 25182654 
47. Cherdtrakulkiat R, Boonpangrak S, Sinthupoom N, Prachayasittikul S, 
Ruchirawat S, Prachayasittikul V. Derivatives (halogen, nitro and amino) 
of 8-hydroxyquinoline with highly potent antimicrobial and antioxidant 
activities. Biochem Biophys Rep. 2016;6:135-41. DOI: 10.1016/j.
bbrep.2016.03.014 PMID: 29214226 
48. Pelletier C, Prognon P, Bourlioux P. Roles of divalent cations and pH 
in mechanism of action of nitroxoline against Escherichia coli strains. 
Antimicrob Agents Chemother. 1995;39(3):707-13. DOI: 10.1128/
AAC.39.3.707 PMID: 7793877 
49. Oviedo P, Quiroga M, Pegels E, Husulak E, Vergara M. Effects of 
subinhibitory concentrations of ciprofloxacin on enterotoxigenic 
Escherichia coli virulence factors. J Chemother. 2000;12(6):487-90. DOI: 
10.1179/joc.2000.12.6.487 PMID: 11154030 
50. Fraser RS, Creanor J. Rapid and selective inhibition of RNA synthesis 
in yeast by 8-hydroxyquinoline. Eur J Biochem. 1974;46(1):67-73. DOI: 
10.1111/j.1432-1033.1974.tb03597.x PMID: 4854024 
51. Sobke A, Klinger M, Hermann B, Sachse S, Nietzsche S, Makarewicz O, et al. 
The urinary antibiotic 5-nitro-8-hydroxyquinoline (Nitroxoline) reduces 
the formation and induces the dispersal of Pseudomonas aeruginosa 
biofilms by chelation of iron and zinc. Antimicrob Agents Chemother. 
2012;56(11):6021-5. DOI: 10.1128/AAC.01484-12 PMID: 22926564 
52. Mrhar A, Kopitar Z, Kozjek F, Presl V, Karba R. Clinical pharmacokinetics 
of nitroxoline. Int J Clin Pharmacol Biopharm. 1979;17(12):476-81. PMID: 
118941 
53. Wagenlehner FM, Münch F, Pilatz A, Bärmann B, Weidner W, Wagenlehner 
CM, et al. Urinary concentrations and antibacterial activities of nitroxoline 
at 250 milligrams versus trimethoprim at 200 milligrams against 
uropathogens in healthy volunteers. Antimicrob Agents Chemother. 
2014;58(2):713-21. DOI: 10.1128/AAC.02147-13 PMID: 24217699 
54. Wijma RA, Huttner A, Koch BC, Mouton JW, Muller AE. Review of 
the pharmacokinetic properties of nitrofurantoin and nitroxoline. J 
Antimicrob Chemother. 2018;73(11):2916-26. DOI: 10.1093/jac/dky255 
PMID: 30184207 
55. Shim JS, Matsui Y, Bhat S, Nacev BA, Xu J, Bhang HE, et al. Effect of 
nitroxoline on angiogenesis and growth of human bladder cancer. J Natl 
Cancer Inst. 2010;102(24):1855-73. [cited 2013 Nov 18]. DOI: 10.1093/jnci/
djq457 PMID: 21088277 
56. Xu N, Lin W, Sun J, Sadahira T, Xu A, Watanabe M, et al. Nitroxoline inhibits 
bladder cancer progression by reversing EMT process and enhancing 
anti-tumor immunity. J Cancer. 2020;11(22):6633-41. DOI: 10.7150/
jca.47025 PMID: 33046984 
57. Mirković B, Renko M, Turk S, Sosič I, Jevnikar Z, Obermajer N, et al. 
Novel mechanism of cathepsin B inhibition by antibiotic nitroxoline and 
related compounds. ChemMedChem. 2011;6(8):1351-6. DOI: 10.1002/
cmdc.201100098 PMID: 21598397 
58. Mirković B, Markelc B, Butinar M, Mitrović A, Sosič I, Gobec S, et al. 
Nitroxoline impairs tumor progression in vitro and in vivo by regulating 
cathepsin B activity. Oncotarget. 2015;6(22):19027-42. 
59. Vasiljeva O, Turk B. Dual contrasting roles of cysteine cathepsins in 
cancer progression: apoptosis versus tumour invasion. Biochimie. 
2008;90(2):380-6. DOI: 10.1016/j.biochi.2007.10.004 PMID: 17991442 
60. Gocheva V, Zeng W, Ke D, Klimstra D, Reinheckel T, Peters C, et al. Distinct 
roles for cysteine cathepsin genes in multistage tumorigenesis. Genes 
Dev. 2006;20(5):543-56. DOI: 10.1101/gad.1407406 PMID: 16481467 
61. Chan-On W, Huyen NT, Songtawee N, Suwanjang W, Prachayasittikul S, 
Prachayasittikul V. Quinoline-based clioquinol and nitroxoline exhibit 
anticancer activity inducing FoxM1 inhibition in cholangiocarcinoma 
cells. Drug Des Devel Ther. 2015;9:2033. PMID: 25897210 
62. Mitrović A, Kos J. Nitroxoline: repurposing its antimicrobial to antitumor 
application. Acta Biochim Pol. 2019;66(4):521-31. PMID: 31834689 
63. Veschi S, De Lellis L, Florio R, Lanuti P, Massucci A, Tinari N, et al. Effects of 
repurposed drug candidates nitroxoline and nelfinavir as single agents or 
in combination with erlotinib in pancreatic cancer cells. J Exp Clin Cancer 
Res. 2018;37(1):236. DOI: 10.1186/s13046-018-0904-2 PMID: 30241558 
64. Sosič I, Mirković B, Arenz K, Štefane B, Kos J, Gobec S. Development of 
new cathepsin B inhibitors: combining bioisosteric replacements and 
structure-based design to explore the structure-activity relationships 
of nitroxoline derivatives. J Med Chem. 2013;56(2):521-33. DOI: 10.1021/
jm301544x PMID: 23252745 
65. Sosič I, Mitrović A, Ćurić H, Knez D, Brodnik Žugelj H, Štefane B, et al. 
Cathepsin B inhibitors: further exploration of the nitroxoline core. Bioorg 
Med Chem Lett. 2018;28(7):1239-47. DOI: 10.1016/j.bmcl.2018.02.042 
PMID: 29503024 
66. Mitrović A, Sosič I, Kos Š, Tratar UL, Breznik B, Kranjc S, et al. Addition 
of 2-(ethylamino)acetonitrile group to nitroxoline results in significantly 
improved anti-tumor activity in vitro and in vivo. Oncotarget. 
2017;8(35):59136-47. DOI: 10.18632/oncotarget.19296 PMID: 28938624 
67. Bhat S, Shim JS, Zhang F, Chong CR, Liu JO. Substituted oxines inhibit 
endothelial cell proliferation and angiogenesis. Org Biomol Chem. 
2012;10(15):2979-92. DOI: 10.1039/c2ob06978d PMID: 22391578 
68. Chen W, Zhang H, Chen Z, Jiang H, Liao L, Fan S, et al. Development and 
evaluation of a novel series of Nitroxoline-derived BET inhibitors with 
antitumor activity in renal cell carcinoma. Oncogenesis. 2018;7(11):83. 
DOI: 10.1038/s41389-018-0093-z PMID: 30385738 
69. Xing J, Zhang R, Jiang X, Hu T, Wang X, Qiao G, et al. Rational design 
of 5-((1H-imidazol-1-yl)methyl)quinolin-8-ol derivatives as novel 
bromodomain-containing protein 4 inhibitors. Eur J Med Chem. 
2019;163:281-94. DOI: 10.1016/j.ejmech.2018.11.018 PMID: 30529546 
70. Mitrović A, Kljun J, Sosič I, Uršič M, Meden A, Gobec S, et al. 
Organoruthenated Nitroxoline Derivatives Impair Tumor Cell Invasion 
through Inhibition of Cathepsin B Activity. Inorg Chem. 2019;58(18):12334-
47. DOI: 10.1021/acs.inorgchem.9b01882 PMID: 31464130 
225
REVIEW ARTICLE
Possible use of antimicrobial agents in cancer therapy
71. Hu D, Xu H, Xiao B, Li D, Zhou Z, Liu X, et al. Albumin-Stabilized Metal-
Organic Nanoparticles for Effective Delivery of Metal Complex Anticancer 
Drugs. ACS Appl Mater Interfaces. 2018;10(41):34974-82. DOI: 10.1021/
acsami.8b12812 PMID: 30238746 
72. Oliphant CM, Green GM. Quinolones: a comprehensive review. Am Fam 
Physician. 2002;65(3):455-64. PMID: 11858629 
73. Dalhoff A. Immunomodulatory activities of fluoroquinolones. Infection. 
2005;33(S2):55-70. DOI: 10.1007/s15010-005-8209-8 PMID: 16518713 
74. Herold C, Ocker M, Ganslmayer M, Gerauer H, Hahn EG, Schuppan D. 
Ciprofloxacin induces apoptosis and inhibits proliferation of human 
colorectal carcinoma cells. Br J Cancer. 2002;86(3):443-8. DOI: 10.1038/
sj.bjc.6600079 PMID: 11875713 
75. Aranha O, Grignon R, Fernandes N, McDonnell TJ, Wood DP, Sarkar FH. 
Suppression of human prostate cancer cell growth by ciprofloxacin 
is associated with cell cycle arrest and apoptosis. Int J Oncol. 
2003;22(4):787-94. DOI: 10.3892/ijo.22.4.787 PMID: 12632069 
76. Phiboonchaiyanan PP, Kiratipaiboon C, Chanvorachote P. Ciprofloxacin 
mediates cancer stem cell phenotypes in lung cancer cells through 
caveolin-1-dependent mechanism. Chem Biol Interact. 2016;250:1-11. 
DOI: 10.1016/j.cbi.2016.03.005 PMID: 26947806 
77. Chen TC, Hsu YL, Tsai YC, Chang YW, Kuo PL, Chen YH. Gemifloxacin 
inhibits migration and invasion and induces mesenchymal-epithelial 
transition in human breast adenocarcinoma cells. J Mol Med (Berl). 
2014;92(1):53-64. DOI: 10.1007/s00109-013-1083-4 PMID: 24005829 
78. Kan JY, Hsu YL, Chen YH, Chen TC, Wang JY, Kuo PL. Gemifloxacin, a 
fluoroquinolone antimicrobial drug, inhibits migration and invasion 
of human colon cancer cells. BioMed Res Int. 2013;2013:159786. DOI: 
10.1155/2013/159786 PMID: 24386633 
79. Gong JH, Liu XJ, Shang BY, Chen SZ, Zhen YS. HERG K+ channel related 
chemosensitivity to sparfloxacin in colon cancer cells. Oncol Rep. 
2010;23(6):1747-56. PMID: 20428834 
80. Shan G, Li Y, Zhang J, Li W, Szulwach KE, Duan R, et al. A small molecule 
enhances RNA interference and promotes microRNA processing. Nat 
Biotechnol. 2008;26(8):933-40. DOI: 10.1038/nbt.1481 PMID: 18641635 
81. Valianatos G, Valcikova B, Growkova K, Verlande A, Mlcochova J, Radova 
L, et al. A small molecule drug promoting miRNA processing induces 
alternative splicing of MdmX transcript and rescues p53 activity in human 
cancer cells overexpressing MdmX protein. PLoS One. 2017. ;12(10)DOI: 
10.1371/journal.pone.0185801 PMID: 28973015 
82. Katsarou ME, Efthimiadou EK, Psomas G, Karaliota A, Vourloumis D. Novel 
copper(II) complex of N-propyl-norfloxacin and 1,10-phenanthroline 
with enhanced antileukemic and DNA nuclease activities. J Med Chem. 
2008;51(3):470-8. DOI: 10.1021/jm7013259 PMID: 18205294 
83. Hu G, Yang Y, Yi L, Wang G, Duan N, Wen X, et al. Design, synthesis and 
antitumor activity of C3/C3 bis-fluoroquonolones cross-linked with 
[1,2,4]triazolo[3,4-b] [1,3,4]thiadiazole. Acta Pharm Sin B. 2011;1(3):172-
7. DOI: 10.1016/j.apsb.2011.07.001