Selective Inhibition of Protein Kinase D2 Activity Reduces Human Neutrophil Survival

Small molecule anti-cancer drugs can cause adverse effects; most commonly low blood cell counts. Protein kinase D2 (PKD2) is highly expressed in neutrophils, and this study investigated the role of PKD2 inhibition on the survival of human neutrophils using small molecule compounds.

The kinase inhibition profiles of two Polo like kinase 1 (PLK1) inhibitors, TAK-960 derivative and Compound A were evaluated using 265 kinase enzyme assays. HL60 cells were differentiated into neutrophil-like cells, followed by siRNA-mediated knockdown of PKD2, and cell viability was assessed. Compounds were tested for their effects on cell viability in differentiated HL60 cells and human neutrophils isolated from healthy donors. Furthermore, correlation between PKD2 inhibition and peripheral white blood cell counts was assessed for 23 difluoropyrimido-diazepinone compounds in nude mice bearing HT-29 human colon cancer cell xenografts.

PKD2 knockdown resulted in reduced cell viability in differentiated HL60 cells. Compound A, which selectively inhibits PKD2, significantly reduced the viability of differentiated HL60 cells and human neutrophils, to a greater extent than TAK-960. In contrast, suppression of PLK3, which is also highly expressed in neutrophils, did not affect cell viability, indicating that PKD2 is critical for neutrophil survival. Moreover, a significant positive correlation was observed between PKD2 inhibition and white blood cell counts in nude mice (Pearson's correlation, r=0.75, p=0.009).

PKD2 plays a crucial role in human neutrophil survival. Minimizing PKD2 inhibition during small-molecule drug design could be essential to reduce the risk of neutropenia in patients under anti-cancer treatment.