Repurposed anti-cancer drugs: the future for anti-infective therapy?

KEYWORDS:

• Multi-drug resistance
• ESKAPE
• bacterial infections
• alkylating agents
• antimetabolites
• gallium nitrate

1. Introduction

Infectious diseases challenge human health. The infectious agents vary from proteins such as prions, simple entities as viruses, unicellular prokaryotic organisms like bacteria, to eukaryotic organisms such as fungi and parasites. Thus, it is imperative to develop new strategies that could contribute to increasing our dwindling antibiotic armamentarium, to address the growing threat of antimicrobial resistance [1]. The scarcity of available options is particularly disadvantageous for the treatment of multi-drug resistant (MDR) bacterial infections. Nowadays, bacterial infections constitute a global threat due to a constant increase of resistance against antibiotics; in fact, recent projections indicate that if no new antibacterials are developed by the year 2050 there will be 10 million deaths per year due to intractable bacterial diseases, becoming the main cause of death. Among infectious bacteria, there is a notable group that often are MDR and even pan-drug resistant and that are responsible for nosocomial infections, this group is known as ESKAPE for the species that constitute it: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species. Current efforts to develop improvements in combating these organisms include the modification of present antibiotics in order to increase their activity and to avoid resistance mechanisms, the discovery or development of new ones, as well as the utilization of alternative approaches such as co-administration of adjuvants that attack antibiotic resistance mechanisms, anti-microbial peptides, biological agents such as bacteriophages and targeting virulence instead of growth [2]. However, bringing a new drug to the clinic takes decades, and hence a way to accelerate the implementation of efficient antimicrobials is drug repurposing. In this regard, several drugs in clinical use have been tested as antibacterials and some of them, especially anti-cancer drugs have shown promising antibacterial activity and successful outputs in clinical trials in humans, and therefore are a suitable option for their eventual implementation in anti-infective therapies [3].

2. Anti-cancer drugs

Cancer chemotherapy began in 1942 when based on mustard gas the first cytotoxic compounds used for therapeutic purposes were synthesized and evaluated [4], and then many compounds with different targets were developed. Antimetabolites are structurally similar to essential metabolites, but the body cannot use them. Some common classes of antimetabolites are folate antagonists (methotrexate) [5], purine antagonists (6-mercaptopurine) [6], and pyrimidine antagonists (5-fluorouracil) [7]. Some natural products have been also used with success, such as the vinca alkaloids and taxol [8,9]. In 1978 the FDA approved cisplatin, a DNA alkylating agent, to treat ovarian cancer. Over time, cisplatin became a first-line drug for the treatment of several cancers; nowadays it is known that in addition to its effect on DNA, it also generates reactive oxygen species, increasing its cytotoxicity [10]. From 2001 to 2004, the molecularly targeted therapy began. This fact indicated a paradigm shift in the synthesis of anti-cancer drugs since it was sought to inhibit overexpressed enzymes in cancerous processes. For example, imatinib is the first tyrosine kinase inhibitor available for clinical use. Its main action is to selectively block cell proliferation and induce apoptosis in cells that express the Philadelphia chromosome and house the Bcr-Abl tyrosine kinase, a cause of chronic myeloid leukemia. Recently, substances derived from living organisms, or laboratory-produced versions of such substances have been used to treat cancer. This therapy is known as biological therapy [11]. Among the biological therapies are those collectively known as ‘immunotherapy’ [12].

3. The use of anti-cancer drugs to treat bacterial infections

Cancer cells and bacterial pathogens are similar in some respects; both show fast proliferation and high metabolic rates, ability to disseminate to other tissues, influence of cell-cell communication in their behavior, resistance against therapeutic agents and dependence on an active immune system for their elimination. Moreover, bacterial pathogens often form biofilms that resemble solid tumors in their heterogeneity of microenvironments and cellular states. Hence, it is expected that at least some drugs specifically designed to inhibit cancer progression are also functional as antimicrobials, as it has been found for several anti-cancer compounds. Among anti-cancer drugs, some DNA-alkylating agents such as mitomycin C and cisplatin had shown remarkable antibacterial activities against S. aureus, A. baumannii, and P. aeruginosa, including MDR strains. Importantly these agents are also effective to kill dormant persister cells that are intrinsically tolerant to regular antibiotics, and are effective in animal infection models (Caenhorhabditis elegans and Galleria mellonella) [13,14]. Another remarkable alkylating agent is 3-bromopyruvate, a preclinical anti-cancer drug with selective bactericide activity against S. aureus, including methicillin resistant strains (MRSA), able to disrupt its biofilms and to kill metabolically inactive bacteria [15]; moreover, it is also a covalent inhibitor of the New Delhi metallo-β-lactamase-1 (NDM-1), able to decrease the minimal inhibitory concentration (MIC) for several β-lactams in E. coli strains expressing NDM-1 [16]. A potent antifungal activity against Cryptococcus neoformans has been also reported for 3-bromopyruvate [17]. Other alkylating anti-cancer drugs with anti-bacterial activity include busulfan, carmustine, chlorambucil, diaziquone, lomustine, mechlorethamine, streptozotocin, and thioTEPA [3]. A second important group of anti-cancer compounds that inhibit bacterial proliferation are antimetabolites, among them 5-fluorouracil (5-FU) has well-documented biofilm and virulence inhibition activities due to its interference with bacterial quorum sensing (QS) [18], and was also effective inhibiting bacterial and fungal colonization of central venous catheters in clinical trials.

One of the most promising anti-cancer drugs to become repurposed as an antibacterial is gallium nitrate, a non-redox iron (III) analog used for hypercalcemia of malignancy, which has broad antimicrobial activity against parasites such as trypanosomes, the ESKAPE bacteria P. aeruginosa, and A. baumannii, and other important bacterial pathogens such as Mycobacterium tuberculosis, being effective both in vitro and in vivo in diverse animal infection models including murine pulmonary infections. Moreover, clinical trials in infected cystic fibrosis patients demonstrated that the intravenous (IV) administration of Ga(NO₃)₃ significantly improved the respiratory function and decreased the bacterial load, due to its ability to interfere with iron transport and to disrupt several iron containing enzymes involved in ROS detoxification, respiration, DNA synthesis, etc. [19]. The great majority of clinical P. aeruginosa strains, including those MDR, are sensitive against Ga(NO₃)₃. However, the occurrence and selection of Ga(NO₃)₃ resistance rates are comparable with that of conventional antibiotics such as colistin, ciprofloxacin, and tobramycin, being the main resistance mechanism a moderate decrease in its internalization [19,20].

Beyond cytotoxic drugs, the inhibitor of human kinases sorafenib, which has multiple targets in tumor cells and vasculature, including the RAF/MEK/ERK pathway, used for the treatment of hepatocellular carcinoma, inhibits the growth of MRSA strains; moreover an optimized derivative (PK150) was efficient against an MRSA infection in mice bloodstream, reducing bacterial load 100-fold. These compounds act through a new mechanism involving stimulation of the signal peptidase SpsB which regulates protein secretion, leading to an increase in the secretion of peptidoglycan hydrolase (PGH)-domain-containing proteins and hence promoting autolysis. Furthermore, they kill persister cells, and the continuous exposure of S. aureus to PK150 during 27 daily passes failed to induce resistance, making this drug a robust antibacterial [21].

In addition, similar to cancer pathogenesis, microbial agents successfully shape a hospitable environment by modulating host metabolism to supply their nutritional requirements, while preventing host defenses by disturbing regulatory mechanisms [22]. Therefore, one of the main links between infections and carcinogenesis comes from the interactions between the immune pathways involved in protection. In an oncogenic context, inflammation is known as one of the hallmarks, whereas exposure to microbial components such as endotoxin also evokes an acute inflammatory response. Moreover, during both chronic infections or advanced cancer stages, the persistent antigen exposure to antigen-reactive T-cells leads to cellular exhaustion [23]. In this regard, the increased expression of CTLA-4 and PD-1, two canonical immune-checkpoint receptors involved in T-cell exhaustion, is considered characteristic of cancer immune dysregulation, and potential benefits of anti-PD-1 therapy are now considered for tuberculosis, HIV, and viral hepatitis infections [24]. Overall, several drugs expanding T-cell response that had been developed for cancer therapy are also likely to be efficacious to treat patients with chronic infectious diseases [25].

Alkylation is perhaps the common action mechanism in anti-cancer molecules with antimicrobial activity; however, molecules such as tamoxifen citrate enhance the host immune system to create traps for bacteria allowing neutrophils to attack them. Also imatinib mesylate cooperates with macrophages to kill pathogens such as M. tuberculosis. These two examples show that repurposing of anticancer molecules works in several ways.

The above examples are promising candidates for repurposing anti-cancer drugs to fight against bacterial infections. In this process, the already available information about toxicity, formulation, pharmacology, and escalation of production will significantly reduce time and cost. Similar molecules that failed to reach approval due to low efficacy during clinical trials could be revived as antimicrobials. However, the drug repurposing path faces challenges like the lack of regulatory guidelines for repurposing drug candidates, absence of financial motivation to investigate on drugs with expired patents, or uncertainty about the return of investment for drugs repurposed to treat rare diseases [26].

Several anti-cancer drugs impair nucleic acid metabolism by crosslinking adjacent bases or being intercalated between DNA strands, inhibiting topoisomerases or the nucleotide producing pathways. For these drugs to be active against pathogenic bacteria an effective intracellular concentration must be reached and they must be properly activated, which is dependent on membrane permeability, the affinity of transporters, enzymes, and efflux pumps. Differences in these parameters between the human and bacterial enzymes may influence the efficacy of the repurposing attempt. Moreover, some of the approved anticancer compounds target processes absent in prokaryotic cells, but still show antibacterial activity, for example, inhibitors of signal transduction like sorafenib, gefitinib, ibrutinib or imatinib or modulators of hormone effects like raloxifene, toremifene, or tamoxifen [2]. These drugs may directly bind bacterial proteins or act through a host-mediated response [2]. Thus, drug repurposing will certainly benefit from pre-clinical research to characterize the mechanisms of action in bacteria.

4. Drug repurposing may boost anti-virulence therapy

Regardless of the potential benefits of anti-cancer drugs for the treatment of bacterial infections, if the repurposed drug shows bactericidal activity, the selective pressure may lead to appearance of resistance and thus, the benefit to the clinic may be temporary. One of the promising strategies to overcome the appearance of resistance to antimicrobials is inhibition of virulence without compromising bacterial cell viability, preventing the infection process and host damage, and relying on the assumption that they will eventually be removed by the immune system. If viability is not compromised, in theory, the appearance of resistance is more unlikely [27]. Nevertheless, a possible drawback for anti-virulence drugs is that some clinical strains are not susceptible [28,29], and at least in vitro sporadic resistance against some of them, including brominated furanones, 5-FU and flucytosine have been found [27,28,30,31], hence its eventual utilization should include monitoring its effectivity against the specific strains before and during treatment. The aim of anti-virulence drugs is to impair processes like secretion of exotoxins, biofilm formation, adherence, evasion of the immune system, quorum sensing, etc. [32]. To our knowledge no drug currently used in the clinic has been approved as anti-virulence as its primary use. However, several candidates for drug repurposing have been reported [2]. Among the most promising are the above mentioned anticancer drugs mitomycin C, which inhibits biofilm formation in E. coli, S. aureus, and P. aeruginosa, and 5-FU, which inhibits biofilm formation in E. coli, P. aeruginosa, and S. epidermidis, and represses QS and virulence in P. aeruginosa [2]. Other anti-cancer drugs that efficiently prevented biofilm formation are azacitidine (in S. pneumoniae), toremifene (in S. aureus), aminolevulinic acid (in S. aureus and S. epidermidis), while raloxifene inhibited pyocyanin production in P. aeruginosa, besides, it showed virulence reduction in an in vivo model of Caenorhabditis elegans [2]. The urgent need of new approaches to deal with the rise of antimicrobial resistance demands that industry and academia combine efforts to fully explore the potential of anti-virulence therapy and drug repurposing synergy.

5. Concluding remarks

The examples presented here demonstrate that several anti-cancer drugs have anti-bacterial activity against important bacterial pathogens; however most studies only evaluate their effects in vitro, hence we encourage the community to further advance their studies in suitable animal infection models, in order to identify candidates for clinical trials. To date, drugs like 5-FU and gallium nitrate already showed encouraging results, as a coating for the prevention of microbial colonization of catheters in the case of 5-FU, and to improve the respiratory function of cystic fibrosis patients in the case of gallium, nevertheless they are still not used in the clinic to prevent or treat bacterial infections. In the case of Ga, further clinical trials with a higher number of patients are expected; also a possible way to improve its action and decrease potential side effects may be its administration by inhalation instead of intravenously, so hopefully this also will be evaluated.

Despite the potential benefits of repurposing anti-cancer drugs as antibacterials, drawbacks of these drugs are that they are much more toxic compared to most antibiotics and that they have severe side effects, including immunosuppression. Hence, their administration should be optimized in order to find effective doses to maximize their beneficial effects. One crucial difference between anti-cancer and anti-bacterial therapies is that cancer patients often receive high doses of anti-cancer drugs during several rounds of chemotherapy which last months; however, it is expected that their administration as antimicrobials could be done only for a few days as usually antibiotics are administered; hence, it can be anticipated that the side effects will be milder in infected patients. We propose that these issues have to be exhaustively explored in animal infection models testing their systemic and local applications, e.g., inhaled for pulmonary infections or topical for skin and burn infections. Moreover, a still poorly explored field is to study their interaction with antibiotics in order to find synergistic combinations at lower anti-cancer drug doses. Beyond antibacterial activity, there are also interesting crosslinks between anti-viral and anti-cancer drugs such as ribavirin [33] and between anti-parasitic and anti-cancer drugs such as ivermectin [34], thus it is possible that some anti-cancer drugs have anti-viral and anti-parasitic activities as well.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

The authors are funded by PAPITT-UNAM, CONACYT, Hospital Infantil and SEP-PRODEP. The sponsors had no input to the manuscript.