Antimicrobial peptides (AMPs) are a pervasive and evolutionarily ancient component of innate host defense which is present in virtually all classes of life. In recent years, evidence has accumulated that parallel or de novo mechanisms by which AMPs curb infectious pathologies are also effective at restraining cancer cell proliferation and dissemination, and have consequently stimulated significant interest in their deployment as novel biologic and immunotherapeutic agents against human malignancies. In this review, we explicate the biochemical underpinnings of their tumor-selectivity, and discuss results of recent clinical trials (outside of oncologic indications) which substantiate their safety and tolerability profiles. Next, we present evidence for their preclinical antitumor activity, systematically organized by the major and minor classes of natural AMPs. Finally, we discuss the barriers to their clinical implementation and envision directions for further development.
AMPs and Cancer Treatment Henceforth in this review, we have defined classes of AMPs as “major” or “minor,” respectively, depending on whether their anticancer properties have been the subject of intensive study or are less well described (Figure 3).
Defensins The defensins are cationic peptides produced by eukaryotes and comprise two superfamilies which have undergone divergent evolution in terms of sequence, structure, and function. The anticancer properties of human defensins are featured in a rapidly growing body of findings, and encouraging preclinical results have been obtained with the treatment of cancer cells or xenograft models with various natural or synthetic defensins. For instance, it has been shown that natural human ß-defensin-3 (hBD-3) is capable of suppressing VEGF-induced cancer cell migration capabilities (43–46). In another study, hBD-3 were shown to be produced by tumor-infiltrating monocytes and inhibited the invasiveness and motility of colon cancer cells in a dose-dependent and paracrine fashion (45). Considering that these in vitro malignant phenotypic traits correlate with a primary tumor’s propensity to establish life-threatening metastatic outgrowths, the finding that defensins are effective at repressing cancer cell motility suggests that they can be developed as potential antimetastatic agents (Figure 4).
Furthermore, natural defensins appear to exert antiproliferative and proapoptotic effects on cancer cells and to induce cell cycle arrest (44, 47–51), which are evidenced by increases in the levels of phosphorylated retinoblastoma protein, suppressed activities of transcriptional and cell cycle cyclin-dependent kinases and their catalytic cyclin partners (52), and enhanced expression of caspase 7 and 9 and other markers of apoptosis. Interestingly, human beta-defensin-2 (hBD-2) have been shown to also reduce the viability of melanoma cells through the downregulation of BRAF (52). Besides their natural derivatives, synthetic defensin analogs may be designed for greater anticancer efficacy: Du et al. demonstrated that recombinant tailored defensin (DF-HSA) comprising human ß-defensin-2 (DF) and human serum albumin (HSA) was more effective than natural ß-defensin at curbing the proliferation of K-Ras-mutant MIA PaCa-2 cells and suppressing the growth of a pancreatic carcinoma xenograft (53).
Two additional facets of human defensins warrant discussion: first, it is notable that they appear to have an impressive level of specificity for tumor cells, yet do not appear to exert palpable cytotoxic or cytostatic effects against normal untransformed cells (48, 50, 51, 54). It has been shown that defensins induce apoptosis in MCF-7 cells via the intrinsic pathway, enhanced MAPK p38 phosphorylation, as well as increased expression of cytochrome c, Apaf-1, caspase 7 and 9, but did not affect the membrane potential and calcium flow (48). Another study indicated that Laterosporulin10, a defensin-like anticancer bacteriocin, results in apoptotic and necrotic death of MCF-7, HEK293T, HT1080, HeLa, and H1299 cells (50).
This observation is arguably consistent with the fact that human AMPs are endogenously derived, and therefore are designed to avoid causing overt collateral toxicity to normal healthy tissues during an inflammatory response. Second, antimicrobial defensins may present novel opportunities to address unmet clinical issues such as chemotherapeutic resistance. For instance, defensins have been shown to potentiate cancer cell-kill in combination with cytotoxic chemotherapeutic agents such as doxorubicin in multidrug-resistant cancer cells (51, 54).
Another compelling application for defensins is their significant potential to augment the effectiveness of cancer immunotherapy. Li et al. for instance employed a recombinant plasmid which expresses beta-defensin 2 and evaluated its potential as both cancer gene therapy and immunotherapy (55). In vitro and in vivo results indicated that physiological changes occurred in immature dendritic cells in a fashion which is likely to enhance adaptive anticancer immunosurveillance (55).
Magainin II (MG2) Magainin II (MGN-II) is an AMP isolated from the skin of the African clawed frog Xenopus laevis which has demonstrated potent anticancer effects in various hematopoietic and solid malignancies. As an ionophoric peptide with a helical structure, it has been shown to perforate cancer cell membranes to act as ion channels, causing cytolysis of cancerous cells (64). MGN-II greatly enhances the tumoricidal effects of cytotoxic chemotherapeutic agents. For instance, magainin A (MAG A) and magainin G (MAG G) have been shown to have synergistic effects when used with chemotherapy against non-small cell lung cancer cell lines (65). MGN-II also displays tumor-selectivity; for instance, they have been demonstrated to lyse various hematopoietic tumor and solid tumor cells but have little or no effect on normal human fibroblasts and peripheral blood lymphocytes (8, 10, 12).
Derivatives of MGN-II have also been synthesized to enhance their cancer-specific cytotoxic properties (18, 64, 66). For instance, a magainin II-bombesin conjugate (MG2B) has demonstrated enhanced activity at a lower dose against a wide range of human cancer cells, without adverse effects on normal cells, as well as potent antitumor activity in a murine model of breast cancer (66). In yet another example, the fusion peptide MG2A, which was synthesized by conjugating MGN-II to the NH2-terminal of the cell-penetrating peptide penetratin (Antp), exhibited augmented cytotoxicity against a variety of human cancer cells and rat glioma cells, while having very limited off-target effects on normal cells (18).
Conclusion and Future Directions Therapeutic resistance and metastatic dissemination are some of the most clinically challenging aspects of cancer. There is now an abundance of evidence that AMPs hold substantial potential for filling these therapeutic voids in clinical oncology. In this review, we have discussed evidence for their antiproliferative, proapoptotic, and antimetastatic effects on cancer cells. Crucially, AMPs have also shown activity against multidrug-resistant cancer cell lines, and therefore could prove valuable for the treatment of advanced, refractory cancers. Hence, we envision that in the future, combination strategies involving this novel therapeutic class with conventional cancer treatments (targeted therapies, immunotherapies, and chemotherapy) may improve treatment outcomes.
Nevertheless, significant challenges lie ahead in the path toward their clinical development and deployment. Toxicity continues to feature as a prominent concern, especially with regards to the administration of non-human natural or synthetic AMPs. However, lessons can be learnt from the field of infectious diseases, where several AMPs have transitioned to clinical trials or have even gained a foothold in clinical care. One area in which the therapeutic index of these peptides can be improved is through the development of innovative formulations and drug delivery systems. Structure–activity relationship, lead identification, and optimization studies are also crucial to improving the anticancer efficacy of AMPs (Figure 5). Yet, these will also require greater understanding of their underlying anti-tumorigenic mechanisms, which hitherto remain somewhat speculative. It is also worth noting that the vast majority of AMPs that exist in nature have yet to be characterized, and given their potential for therapeutic applications, there are certainly compelling reasons to conduct more comprehensive surveys to identify drug candidates. However, with focused efforts to overcome these limitations and obstacles, and a rigorous commitment to translate—at a reasonable cost—these promising peptides into cancer care, the future for this emerging therapeutic class looks exceptionally bright.
——————
“Tumor cell membrane-targeting cationic antimicrobial peptides: novel insights into mechanisms of action and therapeutic prospects” (2017)
