Synthesis and growth-inhibitory activities of imidazo[5,1-d]-1,2,3,5-tetrazine-8-carboxamides related to the anti-tumour drug temozolomide, with appended silicon, benzyl and heteromethyl groups at the 3-position

Discovery of new imidazotetrazinones with potential to overcome tumor resistance

We describe the design, organic synthesis, and characterization, including X-ray crystallography, of a series of novel analogues of the clinically used antitumor agent temozolomide, together with their in vitro biological evaluation. The work has resulted in the discovery of a new series of anticancer imidazotetrazines that offer the potential to overcome the resistance mounted by tumors against temozolomide. The rationally designed compounds that incorporate a propargyl alkylating moiety and a thiazole ring as isosteric replacement for a carboxamide, are readily synthesized (gram-scale), exhibit defined solid-state structures, and enhanced growth-inhibitory activity against human tumor cell lines, including MGMT-expressing and MMR-deficient lines, molecular features that confer tumor resistance. The cell proliferation data were confirmed by clonogenic cell survival assays, and DNA flow cytometry analysis was undertaken to determine the effects of new analogues on cell cycle progression. Detailed 1H NMR spectroscopic studies showed that the new agents are stable in solution, and confirmed their mechanism of action. The propargyl and thiazole substituents significantly improve potency and physicochemical, drug metabolism and permeability properties, suggesting that the thiazole 13 should be prioritized for further preclinical evaluation.

Novel Imidazotetrazine Evades Known Resistance Mechanisms and Is Effective against Temozolomide-Resistant Brain Cancer in Cell Culture

Glioblastoma (GBM) is the most lethal primary brain tumor. Currently, frontline treatment for primary GBM includes the DNA-methylating drug temozolomide (TMZ, of the imidazotetrazine class), while the optimal treatment for recurrent GBM remains under investigation. Despite its widespread use, a majority of GBM patients do not respond to TMZ therapy; expression of the O⁶-methylguanine DNA methyltransferase (MGMT) enzyme and loss of mismatch repair (MMR) function as the principal clinical modes of resistance to TMZ. Here, we describe a novel imidazotetrazine designed to evade resistance by MGMT while retaining suitable hydrolytic stability, allowing for effective prodrug activation and biodistribution. This dual-substituted compound, called CPZ, exhibits activity against cancer cells irrespective of MGMT expression and MMR status. CPZ has greater blood-brain barrier penetrance and comparable hematological toxicity relative to TMZ, while also matching its maximum tolerated dose in mice when dosed once-per-day over five days. The activity of CPZ is independent of the two principal mechanisms suppressing the effectiveness of TMZ, making it a promising new candidate for the treatment of GBM, especially those that are TMZ-resistant.

C8-Substituted Imidazotetrazine Analogs Overcome Temozolomide Resistance by Inducing DNA Adducts and DNA Damage

Temozolomide (TMZ) is the standard of care chemotherapeutic agent used in the treatment of glioblastoma multiforme. Cytotoxic O6-methylguaine lesions formed by TMZ are repaired by O6-methyl-guanine DNA methyltransferase (MGMT), a DNA repair protein that removes alkyl groups located at the O6-position of guanine. Response to TMZ requires low MGMT expression and functional mismatch repair. Resistance to TMZ conferred by MGMT, and tolerance to O6-methylguanine lesions conferred by deficient MMR severely limit TMZ clinical applications. Therefore, development of new TMZ derivatives that can overcome TMZ-resistance is urgent. In this study, we investigated the anti-tumor mechanism of action of two novel TMZ analogs: C8-imidazolyl (377) and C8-methylimidazole (465) tetrazines. We found that analogs 377 and 465 display good anticancer activity against MGMT-overexpressing glioma T98G and MMR deficient colorectal carcinoma HCT116 cell lines with IC₅₀ value of 62.50, 44.23, 33.09, and 25.37 μM, respectively. Analogs induce cell cycle arrest at G2/M, DNA double strand break damage and apoptosis irrespective of MGMT and MMR status. It was established that analog 377, similar to TMZ, is able to ring-open and hydrolyze under physiological conditions, and its intermediate product is more stable than MTIC. Moreover, DNA adducts of 377 with calf thymus DNA were identified: N7-methylguanine, O6-methylguanine, N3-methyladenine, N3-methylthymine, and N3-methylcytidine deoxynucleotides. We conclude that C8 analogs of TMZ share a mechanism of action similar to TMZ and are able to methylate DNA generating O6-methylguanine adducts, but unlike TMZ are able at least in part to thwart MGMT- and MMR-mediated resistance.