Important aspects of cell cycle regulation are the checkpoints, which respond to a variety of cellular stresses to inhibit cell cycle progression and act as protective mechanisms to ensure genomic integrity. An increasing number of tumor suppressors are being demonstrated to have roles in checkpoint mechanisms, implying that checkpoint dysfunction is likely to be a common feature of cancers. Here we report that histone deacetylase inhibitors, in particular azelaic bishydroxamic acid, triggers a G2 phase cell cycle checkpoint response in normal human cells, and this checkpoint is defective in a range of tumor cell lines. Loss of this G2 checkpoint results in the tumor cells undergoing an aberrant mitosis resulting in fractured multinuclei and micronuclei and eventually cell death. This histone deacetylase inhibitor-sensitive checkpoint appears to be distinct from G2/M checkpoints activated by genotoxins and microtubule poisons and may be the human homologue of a yeast G2 checkpoint, which responds to aberrant histone acetylation states. Azelaic bishydroxamic acid may represent a new class of anticancer drugs with selective toxicity based on its ability to target a dysfunctional checkpoint mechanism in tumor cells.

Summary A series of hydroxamates, which are not metalloprotease inhibitors, have been found to be selectively toxic to a range of transformed and human tumour cells without killing normal cells (fibroblasts, melanocytes) at the same concentrations. Within 24 h of treatment, drug action is characterized by morphological reversion of tumour cells to a more normal phenotype (dendritic morphology), and rapid and reversible acetylation of histone H4 in both tumour and normal cells. Two hydroxamates inhibited growth of xenografts of human melanoma cells in nude mice; resistance did not develop in vivo or in vitro. A third hydroxamate, trichostatin A, was active in vitro but became inactivated and had no anti-tumour activity in vivo. Development of dendritic morphology was found to be dependent upon phosphatase activity, RNA and protein synthesis. Proliferating hybrid clones of sensitive and resistant cells remained sensitive to ABHA, indicating a dominant-negative mechanism of sensitivity. Histone H4 hyperacetylation suggests that these agents act at the chromatin level. This work may lead to new drugs that are potent, and selective anti-tumour agents with low toxicity to normal cells.

To understand the time-course of viraemia and antibody responses to severe acute respiratory syndrome-associated coronavirus (SARS-CoV), RT-PCR and ELISA were used to assay 376 blood samples from 135 SARS patients at various stages of the illness, including samples from patients who were in their early convalescent phase. The results showed that IgM antibodies decreased and became undetectable 11 weeks into the recovery phase. IgG antibodies, however, remained detectable for a period beyond 11 weeks and were found in 100 % of patients in the early convalescent phase. SARS-CoV viraemia mainly appeared 1 week after the onset of illness and then decreased over a period of 1 month, becoming undetectable in the blood samples of the convalescent patients. At the peak of viraemia, viral RNA was detectable in 75 % of blood samples from patients who were clinically diagnosed with SARS 1 or 2 weeks before the test.