Internalization and stability of a thymidylate synthase Peptide inhibitor in ovarian cancer cells

Peptides for diagnosis and treatment of ovarian cancer

Ovarian cancer is the most deadly gynecologic malignancy, and its incidence is gradually increasing. Despite improvements after treatment, the results are unsatisfactory and survival rates are relatively low. Therefore, early diagnosis and effective treatment remain two major challenges. Peptides have received significant attention in the search for new diagnostic and therapeutic approaches. Radiolabeled peptides specifically bind to cancer cell surface receptors for diagnostic purposes, while differential peptides in bodily fluids can also be used as new diagnostic markers. In terms of treatment, peptides can exert cytotoxic effects directly or act as ligands for targeted drug delivery. Peptide-based vaccines are an effective approach for tumor immunotherapy and have achieved clinical benefit. In addition, several advantages of peptides, such as specific targeting, low immunogenicity, ease of synthesis and high biosafety, make peptides attractive alternative tools for the diagnosis and treatment of cancer, particularly ovarian cancer. In this review, we focus on the recent research progress regarding peptides in the diagnosis and treatment of ovarian cancer, and their potential applications in the clinical setting.

Folic Acid-Peptide Conjugates Combine Selective Cancer Cell Internalization with Thymidylate Synthase Dimer Interface Targeting

Drug-target interaction, cellular internalization, and target engagement should be addressed to design a lead with high chances of success in further optimization stages. Accordingly, we have designed conjugates of folic acid with anticancer peptides able to bind human thymidylate synthase (hTS) and enter cancer cells through folate receptor α (FRα) highly expressed by several cancer cells. Mechanistic analyses and molecular modeling simulations have shown that these conjugates bind the hTS monomer-monomer interface with affinities over 20 times larger than the enzyme active site. When tested on several cancer cell models, these conjugates exhibited FRα selectivity at nanomolar concentrations. A similar selectivity was observed when the conjugates were delivered in synergistic or additive combinations with anticancer agents. At variance with 5-fluorouracil and other anticancer drugs that target the hTS catalytic pocket, these conjugates do not induce overexpression of this protein and can thus help combating drug resistance associated with high hTS levels.

Cyclic Peptides Acting as Allosteric Inhibitors of Human Thymidylate Synthase and Cancer Cell Growth

Thymidylate synthase (TS) is a prominent drug target for different cancer types. However, the prolonged use of its classical inhibitors, substrate analogs that bind at the active site, leads to TS overexpression and drug resistance in the clinic. In the effort to identify anti-TS drugs with new modes of action and able to overcome platinum drug resistance in ovarian cancer, octapeptides with a new allosteric inhibition mechanism were identified as cancer cell growth inhibitors that do not cause TS overexpression. To improve the biological properties, 10 cyclic peptides (cPs) were designed from the lead peptides and synthesized. The cPs were screened for the ability to inhibit recombinant human thymidylate synthase (hTS), and peptide 7 was found to act as an allosteric inhibitor more potent than its parent open-chain peptide [Pro³]LR. In cytotoxicity studies on three human ovarian cancer cell lines, IGROV-1, A2780, and A2780/CP, peptide 5 and two other cPs, including 7, showed IC₅₀ values comparable with those of the reference drug 5-fluorouracil, of the open-chain peptide [d-Gln⁴]LR, and of another seven prolyl derivatives of the lead peptide LR. These promising results indicate cP 7 as a possible lead compound to be chemically modified with the aim of improving both allosteric TS inhibitory activity and anticancer effectiveness.

Human Thymidylate Synthase Inhibitors Halting Ovarian Cancer Growth

Human thymidylate synthase (hTS) has an important role in DNA biosynthesis, thus it is essential for cell survival. TS is involved in the folate pathways, specifically in the de novo pyrimidine biosynthesis. Structure and functions are intimately correlated, account for cellular activity and, in a broader view, with in vivo mechanisms. hTS is a target for anticancer agents, some of which are clinical drugs. The understanding of the detailed mechanism of TS inhibition by currently used drugs and of the interaction with the mechanism of action of other anticancer agents can suggest new perspective of TS inhibition able to improve the anticancer effect and to overcome drug resistance. TS-targeting drugs in therapy today are inhibitors that bind at the active site and that mostly resemble the substrates. Nonsubstrate analogs offer an opportunity for allosteric binding and novel mode of inhibition in the cancer cells. This chapter illustrates the relationship among the large number of hTS actions at molecular and clinical levels, its role as a target for ovarian cancer therapy, in particular in cases of overexpression of hTS and other folate proteins such as those induced by platinum drug treatments, and address the potential combination of TS inhibitors with other suitable anticancer agents.

Crystal structures of nematode (parasitic T. spiralis and free living C. elegans), compared to mammalian, thymidylate synthases (TS). Molecular docking and molecular dynamics simulations in search for nematode-specific inhibitors of TS

Three crystal structures are presented of nematode thymidylate synthases (TS), including Caenorhabditis elegans (Ce) enzyme without ligands and its ternary complex with dUMP and Raltitrexed, and binary complex of Trichinella spiralis (Ts) enzyme with dUMP. In search of differences potentially relevant for the development of species-specific inhibitors of the nematode enzyme, a comparison was made of the present Ce and Ts enzyme structures, as well as binary complex of Ce enzyme with dUMP, with the corresponding mammalian (human, mouse and rat) enzyme crystal structures. To complement the comparison, tCONCOORD computations were performed to evaluate dynamic behaviors of mammalian and nematode TS structures. Finally, comparative molecular docking combined with molecular dynamics and free energy of binding calculations were carried out to search for ligands showing selective affinity to T. spiralis TS. Despite an overall strong similarity in structure and dynamics of nematode vs mammalian TSs, a pool of ligands demonstrating predictively a strong and selective binding to TsTS has been delimited. These compounds, the E63 family, locate in the dimerization interface of TsTS where they exert species-specific interactions with certain non-conserved residues, including hydrogen bonds with Thr174 and hydrophobic contacts with Phe192, Cys191 and Tyr152. The E63 family of ligands opens the possibility of future development of selective inhibitors of TsTS and effective agents against trichinellosis.

Intracellular quantitative detection of human thymidylate synthase engagement with an unconventional inhibitor using tetracysteine-diarsenical-probe technology

Demonstrating a candidate drug's interaction with its target protein in live cells is of pivotal relevance to the successful outcome of the drug discovery process. Although thymidylate synthase (hTS) is an important anticancer target protein, the efficacy of the few anti-hTS drugs currently used in clinical practice is limited by the development of resistance. Hence, there is an intense search for new, unconventional anti-hTS drugs; there are approximately 1600 ongoing clinical trials involving hTS-targeting drugs, both alone and in combination protocols. We recently discovered new, unconventional peptidic inhibitors of hTS that are active against cancer cells and do not result in the overexpression of hTS, which is a known molecular source of resistance. Here, we propose an adaptation of the recently proposed tetracysteine-arsenic-binding-motif technology to detect and quantitatively characterize the engagement of hTS with one such peptidic inhibitor in cell lysates. This new model can be developed into a test for high-throughput screening studies of intracellular target-protein/small-molecule binding.

Deoxyribonucleotide metabolism, mutagenesis and cancer

Cancer was recognized as a genetic disease at least four decades ago, with the realization that the spontaneous mutation rate must increase early in tumorigenesis to account for the many mutations in tumour cells compared with their progenitor pre-malignant cells. Abnormalities in the deoxyribonucleotide pool have long been recognized as determinants of DNA replication fidelity, and hence may contribute to mutagenic processes that are involved in carcinogenesis. In addition, many anticancer agents antagonize deoxyribonucleotide metabolism. Here, we consider the extent to which aspects of deoxyribonucleotide metabolism contribute to our understanding of both carcinogenesis and to the effective use of anticancer agents.

Modifying peptides to enhance permeability

Recently, peptides have been validated to address intracellular targets and/or to be orally bioavailable. This review describes some of these scaffolds, offers insight in new cyclization methodologies thought to be beneficial to enhance permeability, and highlights modification on peptides thought to improve oral bioavailability. In this context, side chains and back-bone derivatization beneficial to encourage cellular uptake are presented. In addition, new methodologies supporting the assessment of permeability are discussed.