A pharmacodynamic study of rapamycin in men with intermediate- to high-risk localized prostate cancer

Abstract

Purpose: Given discrepancies between preclinical and clinical observations of mammalian target of rapamycin (mTOR) inhibition in prostate cancer, we sought to determine the pharmacodynamic effects of the mTOR/TORC1 inhibitor rapamycin in men with intermediate- to high-risk prostate cancer undergoing radical prostatectomy.

Experimental design: Rapamycin was given at 3 or 6 mg orally for 14 days before radical prostatectomy in men with multifocal Gleason sum > or =7 prostate cancer; 10 untreated control subjects were included. The primary outcome was inhibition of phosphorylation of ribosomal S6 in posttreatment radical prostatectomy versus pretreatment biopsy tumor tissue, evaluated using a Simon two-stage design for pharmacodynamic efficacy.

Results: Thirty-two subjects were accrued: 20 at 3 mg, 2 at 6 mg, and 10 controls. No dose-limiting toxicities were observed at 3 mg; however, two of two men enrolled at 6 mg experienced dose-limiting toxicities including thrombocytopenia and fever with grade 3 stomatitis. Adverse events observed at 3 mg included stomatitis, rash, ileus, and neutropenia. Pharmacodynamic studies showed tumor S6 phosphorylation inhibition in 50% of 10 evaluable rapamycin-treated men with sufficient paired tissue [median 58% inhibition (P = 0.049) versus 2% inhibition in controls (P = 0.75)] with no significant effect on AKT activity. We observed no change in Ki-67 or caspase-3 cleavage but noted a reduction in cytoplasmic p27 staining with increased nuclear localization with rapamycin treatment. Prostate tissue rapamycin concentrations were 3- to 4-fold higher than blood.

Conclusions: At 3 mg daily, rapamycin successfully and safely inhibited prostate cancer S6 phosphorylation and achieved relatively high prostate tissue concentrations. No effect on AKT phosphorylation, tumor proliferation, or apoptosis was observed.

Figure 1

Left: Box plots and pathologic representation of the primary PD endpoint, S6 kinase inhibition (S6 phosphorylation), in paired prostate tissues (biopsy and radical prostatectomy (RP) cancer tissue) among rapamycin treated and control subjects. Right: Example of immunohistochemical staining of subject 1 pre-treatment biopsy tissue and post-treatment RP specimen, with H-score depicted. Figures shown within boxes represent median values.

Figure 2

Box plot representations of the secondary PD endpoints as measured in paired biopsy and radical prostatectomy (RP) specimens for: A) AKT activity (phosphorylation) change, B) change in proliferation (Ki-67), and C) changes in nuclear cleaved caspase 3 products. Figures shown within boxes represent median values for each biomarker.

Figure 3

Box plots and bar graphs demonstrating: A) change in p27 expression in paired prostate tissues (biopsy and RP specimens) among rapamycin treated and control subjects. Figures shown within boxes represent median values. B) p27 nuclear localization in paired prostate tissues among rapamycin treated and control subjects.

Figure 4

Paired bar plot demonstrating peripheral blood mononuclear cell (PBMC) and prostate tumor phospho-S6 inhibition among those rapamycin-treated subjects (n=9) who had available PBMC and tumor tissue for biomarker assessment. Phospho-S6 percent change (before and after rapamycin) is calculated using the equation depicted in the figure.

Figure 5

Individual rapamycin concentrations in radical prostatectomy tumor tissue (filled symbols, expressed as ng/g) and peripheral blood (open symbols, expressed as ng/g) grouped by subject number (left) and rapamycin dose cohorts (right): 3 mg (circles) or 6 mg per (squares).