Inhibitors of the enzyme poly (ADP-ribose) polymerase, or PARP, are among the most promising drugs currently in development as anti-tumour agents.

However, their utility is severely limited by the fact that they are only effective against tumours in which a DNA repair process called homologous recombination is defective, and in particular, tumours with mutations in the breast cancer susceptibility gene BRCA1.

Only a fairly small fraction of adult solid tumours fall into this category. Co-prescription of PARP inhibitors with agents that inhibit the normal function of BRCA1 would therefore be likely to extend the utility of these candidate drugs.

The enzyme cyclin-dependent kinase 1 (Cdk1) plays an important part in regulating the cell cycle. 

It phosphorylates BRCA1, a process that is necessary if this protein is to form foci at sites of DNA damage, enabling DNA repair to take place. 

Geoffrey Shapiro from the Dana-Farber Cancer Institute and Harvard Medical School, Boston, Massachusetts, USA and his co-workers have now shown that depletion or inhibition of Cdk1 reduces the ability of cells with intact BRCA1 to repair DNA by homologous recombination, thus replicating a BRCA1-deficient phenotype.

Shapiro and his co-workers first mutated the three serine residues in BRCA1 that are phosphorylated by Cdk1 to alanine, so it could not be activated by phosphorylation.

This mutant protein showed a significant decline in ability to respond to DNA damage in tumour cells irradiated by gamma rays.

The same effect was observed when the cells were treated with a small-molecule Cdk1 inhibitor, RO-3306; neither depletion nor inhibition of Cdk1 had any effect on the DNA damage response to gamma radiation, confirming that the deficiency was at the level of the BRCA1 response.

The researchers next assessed the level of homologous recombination directly using an assay in which GFP was expressed only when this process occurred, and showed that GFP expression levels were reduced when Cdk1 was depleted using siRNA.

Next, the researchers treated tumour cells in which Cdk1 had been depleted with the PARP inhibitor AG14361. This caused chromosome abnormalities to accumulate in the cells, which later died. In contrast, this compound had little effect on cells depleted only in the related kinase Cdk2 or on cells in which both kinases had normal expression levels. 

Shapiro and his colleagues then investigated the effect of Cdk1 depletion in vivo by establishing xenografts of tumour cells in which Cdk1 was inhibited on exposure to doxycycline in immunosuppressed mice. 

Tumour growth was delayed in mice both fed a diet containing doxycycline and treated with the PARP inhibitor AG014699, but not in mice given neither or only one of these treatments.

A combination of a PARP inhibitor with a Cdk1 inhibitor also showed reduced tumour growth and prolonged survival compared to treatment with one or no inhibitors in a mouse model of lung adenocarcinoma.

Taken together, these results show that inhibition of Cdk1 is likely to sensitise tumours with wild type BRCA1 to PARP inhibition, and that a combination of inhibitors of these two enzymes may be a useful treatment for these tumours. It is also encouraging that neither of the inhibitors tested showed significant toxicity in the mouse models used.

Reference

Johnson, N., Li, Y-C., Walton, Z.E. and 12 others (2011). Compromised CDK1 activity sensitizes BRCA-proficient cancers to PARP inhibition. Nature Medicine, published online ahead of print 26 June 2011. doi:10.1038/nm.2377