The induction of aneuploidy by alkylated purines: effects on early and late cell cycle events

Preclinical evaluation of AT-527, a novel guanosine nucleotide prodrug with potent, pan-genotypic activity against hepatitis C virus

Despite the availability of highly effective direct-acting antiviral (DAA) regimens for the treatment of hepatitis C virus (HCV) infections, sustained viral response (SVR) rates remain suboptimal for difficult-to-treat patient populations such as those with HCV genotype 3, cirrhosis or prior treatment experience, warranting development of more potent HCV replication antivirals. AT-527 is the hemi-sulfate salt of AT-511, a novel phosphoramidate prodrug of 2’-fluoro-2’-C-methylguanosine-5'-monophosphate that has potent in vitro activity against HCV. The EC₅₀ of AT-511, determined using HCV laboratory strains and clinical isolates with genotypes 1–5, ranged from 5–28 nM. The active 5'-triphosphate metabolite, AT-9010, specifically inhibited the HCV RNA-dependent RNA polymerase. AT-511 did not inhibit the replication of other selected RNA or DNA viruses in vitro. AT-511 was approximately 10-fold more active than sofosbuvir (SOF) against a panel of laboratory strains and clinical isolates of HCV genotypes 1–5 and remained fully active against S282T resistance-associated variants, with up to 58-fold more potency than SOF. In vitro, AT-511 did not inhibit human DNA polymerases or elicit cytotoxicity or mitochondrial toxicity at concentrations up to 100 μM. Unlike the other potent guanosine analogs PSI-938 and PSI-661, no mutagenic O⁶-alkylguanine bases were formed when incubated with cytochrome P450 (CYP) 3A4, and AT-511 had IC₅₀ values ≥25 μM against a panel of CYP enzymes. In hepatocytes from multiple species, the active triphosphate was the predominant metabolite produced from the prodrug, with a half-life of 10 h in human hepatocytes. When given orally to rats and monkeys, AT-527 preferentially delivered high levels of AT-9010 in the liver in vivo. These favorable preclinical attributes support the ongoing clinical development of AT-527 and suggest that, when used in combination with an HCV DAA from a different class, AT-527 may increase SVR rates, especially for difficult-to-treat patient populations, and could potentially shorten treatment duration for all patients.

Safety, pharmacokinetics and antiviral activity of AT-527, a novel purine nucleotide prodrug, in HCV-infected subjects with and without cirrhosis

AT-527 is a novel modified guanosine nucleotide prodrug inhibitor of the hepatitis C virus (HCV) NS5B polymerase, with increased in vitro antiviral activity compared to sofosbuvir and a highly differentiated favorable preclinical profile compared to other anti-HCV nucleoside/nucleotide analogs.

AT-527 is a novel modified guanosine nucleotide prodrug inhibitor of the hepatitis C virus (HCV) NS5B polymerase, with increased in vitro antiviral activity compared to sofosbuvir and a highly differentiated favorable preclinical profile compared to other anti-HCV nucleoside/nucleotide analogs. This was a multiple-part clinical study where multiple ascending doses of AT-527 up to 600 mg (expressed as AT-527 salt form; equivalent to 553 mg free base) once daily for 7 days were evaluated in a randomized, double-blind, placebo-controlled study of treatment-naive, noncirrhotic, genotype 1b, HCV-infected subjects. The highest dose of AT-527 for the same duration was then evaluated in two open-label cohorts of (i) noncirrhotic, genotype 3, HCV-infected subjects and (ii) HCV-infected subjects of any genotype with compensated (Child-Pugh A) cirrhosis. AT-527 was well tolerated for 7 days in all cohorts. At the highest dose tested, mean HCV RNA reductions of up to 2.4 log₁₀ IU/ml occurred within the first 24 h of dosing. Mean maximum reductions observed with 7 days of dosing were 4.4, 4.5, and 4.6 log₁₀ IU/ml in noncirrhotic subjects with HCV genotype 1b, noncirrhotic subjects with HCV genotype 3, and subjects with compensated cirrhosis, respectively. The systemic half-life of AT-273, the nucleoside metabolite considered a surrogate of intracellular phosphates including the active triphosphate, exceeded 20 h, supporting once-daily dosing. In summary, AT-527 demonstrated rapid, potent, dose/exposure-related, and pangenotypic antiviral activity with similar responses between subjects with and without cirrhosis. Exposure-antiviral response analysis identified 550 mg (free base equivalent) as the optimal dose of AT-527. Safety and antiviral activity data from this study warrant continued clinical development of AT-527 dosed once daily. (This study has been registered in the European Union Drug Regulating Authorities Clinical Trials Database [EudraCT] under number 2017-002148-34 and at ClinicalTrials.gov under identifier NCT03219957.)

Fragile sites at the centromere of Chinese hamster chromosomes: a possible mechanism of chromosome loss

On the basis of our previous observations showing that fragile sites (FS) mapped essentially in the centromeric regions of Chinese hamster chromosomes, we consider the possibility that the presence of FS at the centromere might be a source of chromosome loss. In this model a centromeric FS causes a centromeric break giving rise to two chromosome arms which could be lost or maintained with different consequences on the ploidy of daughter cells. To test this hypothesis, Chinese hamster cells have been treated both with N-methyl-N-nitrosourea (MNU), a mutagenic agent which also induces aneuploidy, and vinblastin (VBL), a pure aneugen, used as a control compound, which is supposed not to interact with DNA. The results show that MNU induces the formation of translocated and/or truncated chromosomes, on the contrary VBL is not able to induce chromosome rearrangements. The sites most involved in MNU-induced breaks are the centromeric regions of chromosomes where FS are also present. These breaks cause essentially the loss of one chromosome arm, so that the resulting cells are numerically diploid but presenting partial monosomies. The implications of these results are discussed.

Induction of kinetochore-containing micronuclei by exogenous O6-methylguanine requires conversion of the methylated base to a nucleotide

It has been reported that exogenous alkylated purines, such as O6-methylguanine (O6meG), induce aneuploidy in mammalian cells. It is shown here that the aneugenic effect of O6meG, evidenced by its ability to induce micronuclei in rodent cells, is dependent on its conversion to O6-methyl-guanosine-5'-monophosphate (O6me-5'-GMP) by hypoxanthine-guanine phosphoribosyl transferase (HPRT). This conclusion, in contrast with previous in vitro data showing that O6meG does not seem to be a substrate for HPRT, was based on the following observations: 1) O6meG did not induce micronuclei in HPRT-deficient Chinese hamster cells, but did induce micronuclei in HPRT-proficient cells, and in mouse cells partially or totally deficient in adenine phosphoribosyl transferase; 2) O6meG was not metabolized in HPRT-deficient cells, while in wild-type cells a number of metabolites were detected by high performance liquid chromatography (HPLC) analysis of cold acid extracts, one of them coeluting with O6me-5'-GMP used as a marker; 3) when de novo synthesis of purine nucleotides was inhibited by aminopterin, O6meG sustained the growth of HPRT-proficient, but not of HPRT-deficient, cells; and 4) when HPRT-deficient cells were treated with liposomes charged with O6me-5'-GMP, induction of micronuclei was shown. The finding that methylated guanine exerts its aneugenic action through methylated nucleotide(s) provides an important, though indirect, support to the hypothesis that alkylating agents may induce aneuploidy via nucleotide pool alkylation.