Alanine mutants of the interface residues of human thymidylate synthase decode key features of the binding mode of allosteric anticancer peptides

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.

Targeting the TS dimer interface in bifunctional Cryptosporidium hominis TS-DHFR from parasitic protozoa: Virtual screening identifies novel TS allosteric inhibitors

Effective therapies are lacking to treat gastrointestinal infections caused by the genus Cryptosporidium, which can be fatal in the immunocompromised. One target of interest is Cryptosporidium hominis (C. hominis) thymidylate synthase-dihydrofolate reductase (ChTS-DHFR), a bifunctional enzyme necessary for DNA biosynthesis. Targeting the TS-TS dimer interface is a novel strategy previously used to identify inhibitors against the related bifunctional enzyme in Toxoplasma gondii. In the present study, we target the ChTS dimer interface through homology modeling and high-throughput virtual screening to identifying allosteric, ChTS-specific inhibitors. Our work led to the discovery of methylenedioxyphenyl-aminophenoxypropanol analogues which inhibit ChTS activity in a manner that is both dose-dependent and influenced by the conformation of the enzyme. Preliminary results presented here include an analysis of structure activity relationships and a ChTS-apo crystal structure of ChTS-DHFR supporting the continued development of inhibitors that stabilize a novel pocket formed in the open conformation of ChTS-TS.

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.

Inside the biochemical pathways of thymidylate synthase perturbed by anticancer drugs: Novel strategies to overcome cancer chemoresistance

Our current understanding of the mechanisms of action of antitumor agents and the precise mechanisms underlying drug resistance is that these two processes are directly linked. Moreover, it is often possible to delineate chemoresistance mechanisms based on the specific mechanism of action of a given anticancer drug. A more holistic approach to the chemoresistance problem suggests that entire metabolic pathways, rather than single enzyme targets may better explain and educate us about the complexity of the cellular responses upon cytotoxic drug administration. Drugs, which target thymidylate synthase and folate-dependent enzymes, represent an important therapeutic arm in the treatment of various human malignancies. However, prolonged patient treatment often provokes drug resistance phenomena that render the chemotherapeutic treatment highly ineffective. Hence, strategies to overcome drug resistance are primarily designed to achieve either enhanced intracellular drug accumulation, to avoid the upregulation of folate-dependent enzymes, and to circumvent the impairment of DNA repair enzymes which are also responsible for cross-resistance to various anticancer drugs. The current clinical practice based on drug combination therapeutic regimens represents the most effective approach to counteract drug resistance. In the current paper, we review the molecular aspects of the activity of TS-targeting drugs and describe how such mechanisms are related to the emergence of clinical drug resistance. We also discuss the current possibilities to overcome drug resistance by using a molecular mechanistic approach based on medicinal chemistry methods focusing on rational structural modifications of novel antitumor agents. This paper also focuses on the importance of the modulation of metabolic pathways upon drug administration, their analysis and the assessment of their putative roles in the networks involved using a meta-analysis approach. The present review describes the main pathways that are modulated by TS-targeting anticancer drugs starting from the description of the normal functioning of the folate metabolic pathway, through the protein modulation occurring upon drug delivery to cultured tumor cells as well as cancer patients, finally describing how the pathways are modulated by drug resistance development. The data collected are then analyzed using network/netwire connecting methods in order to provide a wider view of the pathways involved and of the importance of such information in identifying additional proteins that could serve as novel druggable targets for efficacious cancer therapy.

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.

Hotspots in an obligate homodimeric anticancer target. Structural and functional effects of interfacial mutations in human thymidylate synthase

Human thymidylate synthase (hTS), a target for antiproliferative drugs, is an obligate homodimer. Single-point mutations to alanine at the monomer-monomer interface may enable the identification of specific residues that delineate sites for drugs aimed at perturbing the protein-protein interactions critical for activity. We computationally identified putative hotspot residues at the interface and designed mutants to perturb the intersubunit interaction. Dimer dissociation constants measured by a FRET-based assay range from 60 nM for wild-type hTS up to about 1 mM for single-point mutants and agree with computational predictions of the effects of these mutations. Mutations that are remote from the active site retain full or partial activity, although the substrate KM values were generally higher and the dimer was less stable. The lower dimer stability of the mutants can facilitate access to the dimer interface by small molecules and thereby aid the design of inhibitors that bind at the dimer interface.