KX2-391 is an oral non–ATP-competitive inhibitor of Src kinase and tubulin polymerization. In phase 1 trials, prostate-specific antigen (PSA) declines were seen in patients with advanced prostate cancer. We conducted a single-arm phase 2 study evaluating KX2-391 in men with chemotherapy-naïve bone-metastatic castration-resistant prostate cancer (CRPC).
We treated 31 patients with oral KX2-391 (40mg twice-daily) until disease progression or unacceptable toxicity. The primary endpoint was 24-week progression-free survival (PFS); a 50% success rate was predefined as clinically significant. Secondary endpoints included PSA progression-free survival (PPFS) and PSA response rates. Exploratory outcomes included pharmacokinetic studies, circulating tumor cell (CTC) enumeration, and analysis of markers of bone resorption (urinary N-telopeptide [uNTx]; C-telopeptide [CTx]) and formation (bone alkaline phosphatase [BAP]; osteocalcin).
The trial closed early after accrual of 31 patients, due to a prespecified futility rule. PFS at 24 weeks was 8%, and median PFS was 18.6 weeks. The PSA response rate (≥30% decline) was 10%, and median PPFS was 5.0 weeks. Additionally, 18% of men with unfavorable (≥5) CTCs at baseline converted to favorable (<5) CTCs with treatment. The proportion of men with declines in bone turnover markers was 32% for uNTx, 21% for CTx, 10% for BAP, and 25% for osteocalcin. In pharmacokinetic studies, median Cmax was 61 (range 16–129) ng/mL, and median AUC was 156 (35–348) ng*hr/mL. Common toxicities included hepatic derangements, myelosuppression, fatigue, nausea and constipation.
KX2-391 dosed at 40mg twice-daily lacks antitumor activity in men with CRPC, but has modest effects on bone turnover markers. Because a Cmax of ≥142 ng/mL is required for tubulin polymerization inhibition (defined from preclinical studies), higher once-daily dosing will be used in future trials.