Abstract #LB-256: Enzastaurin inhibits in vivo GSK3\#946; phosphorylation in early breast cancer

Enzastaurin is an orally-administered inhibitor of the protein kinase C (PKC) and PI3 kinase (PI3K)/AKT signaling pathways. Signaling through these pathways is implicated in early breast cancer. A single-arm phase II pharmacokinetic-pharmacodynamic study was conducted to confirm the inhibitory activity of enzastaurin on pGSK3\#946;, a critical component of these pathways. Secondary objectives included analysis of other key signaling nodes in the PKC and AKT signaling pathways including pS6 kinase, pCREB, and PKC\#946; II expression pre- and post-treatment. Pre- and post-treatment breast tumor biopsies were obtained after 21-28d 500 mg QD dosing (d1 loading dose of 1125 mg) of enzastaurin for IHC and RNA analysis. Plasma samples were collected after 18 days of dosing to evaluate the steady-state exposures of enzastaurin and its active metabolite. Enzastaurin was well tolerated in the 39 pts enrolled with mostly grade 1 and 2 toxicities except for 1 pt with grade 3 transaminase elevation. Paired samples from 19 pts were suitable for IHC evaluation (had a pre- and post-treatment pGSK3\#946; IHC score). Baseline characteristics included ER (14 positive), PR (9 positive), and Her2 status (2 positive). IHC was performed using Benchmark-XT automated immunostainers with validated assays (Ventana Medical Systems, Tucson, AZ) for pGSK3\#946;, pS6 kinase, pCREB, and PKC\#946; II . IHC was scored by a single pathologist who was blinded to pre/post biopsy samples and clinical information and an H-(histo) score was determined. Enzastaurin significantly decreased pGSK3\#946; ( p = 0.019), pS6 ( p = 0.014), and PKC\#946; II ( p = 0.037) H-scores in the post-treatment biopsies. Steady-state average concentrations for enzastaurin and total analytes (sum of enzastaurin and its metabolite) were 706 nmol/L (94.6% CV) and 1500 nmol/L (52.2% CV), respectively. No correlations between individual patient exposures to enzastaurin and metabolite with pGSK3\#946;, pS6 or PKC\#946; II inhibition were seen, however the exposures achieved were in the range of the target plasma concentrations of 1400 nmol/L for total analytes. Gene expression was analyzed by Affymetrix H133+2 microarrays and compared with genes known to be modulated by enzastaurin in breast cancer cell lines (SKBR3 and MDA-MB231). Although no significant gene changes (FDR [false discovery rate] =.05 or .01 and fold change \#8805; 2) were observed in the overall sample group, common genes were identified as modulated by enzastaurin between the tumors and the breast cancer cell lines (e.g. RARRES3, MMP1, THRSP/SPOT14, STEAP4, TNFSF10Lig). Of interest in the triple negative group (n=2), elevation of death receptor family members was observed and is consistent with other pre-clinical models of enzastaurin activity. This study is the first in vivo, pharmacodynamic demonstration within tumor samples from pts treated with the PKC and AKT pathway inhibitor enzastaurin. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr LB-256.