Actions of the selective protein kinase C inhibitor PKC412 on B-chronic lymphocytic leukemia cells in vitro.

Background And Objectives: The staurosporine derivative PKC412 (CGP41251) is a more selective inhibitor of the conventional isoforms of protein kinase C (PKC) than is the parent compound. In addition to its growth inhibitory properties, PKC412 reverses the efflux function of the multidrug resistance (MDR)-1 gene product, P-glycoprotein (P-gp).

Design And Methods: The in vitro actions of PKC412 were investigated in peripheral blood lymphocytes (PBL) from 4 normal volunteers, B-cell isolates from 3 normal tonsils and 31 patients with B-cell chronic lymphocytic leukemia (B-CLL). Following incubation with PKC412 for 2 days, the viability of B-CLL cells was decreased relative to that of controls (63+/-23% at 1 micromole/L; 52+/-30% at 10 micromole/L; n=20). Normal PBL were significantly more resistant to the drug (91+/-5% viable cells at 1 micromole/L; 73+/-18% at 10 micromole/L; n=4). Thirteen of the B-CLL patients were treated with oral PKC412 in a phase II trial.

Results: PKC activity in malignant cells from these patients showed a reduction post-treatment of 25-96% of their respective pre-treatment levels. Morphologic analysis, as well as in situ assay for DNA strand breaks (TUNEL assay) showed that B-CLL cells were killed by an apoptotic mechanism. In B-CLL cells the mean IC50, for PKC412, as measured by the reduction of 3-(4,5-dimethylthiozol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT), was 2.1 micromol/L in 16 samples in which the IC50 were below the maximum concentration of PKC412 used for the assay. In tonsillar B-cells, the mean IC50 was 11 micromol/L whereas PBL cells were resistant. Four of eight and 1/3 B-CLL samples that were resistant to chlorambucil and fludarabine, respectively, were sensitive to PKC412. In 15/31 B-CLL samples a dose-dependent reversal of P-gp-mediated drug efflux by PKC412 was observed. A statistically significant correlation (p<0.001) was observed between P-gp protein expression as measured by FACScan analysis and the reversal of efflux activity by either PKC412 or verapamil. PKC412 increased the sensitivity of B-CLL cells to 2'-chlorodeoxyadenosine and chlorambucil.

Interpretation And Conclusions: This study establishes the in vitro cytotoxic and multidrug resistance (MDR) modulatory properties of PKC412 towards malignant cells from B-CLL patients. The direct antitumor activity combined with the potential for P-gp modulation make PKC412 an attractive drug for the treatment of malignancies expressing the MDR phenotype, or in combination with conventional drugs.