A phase 2 study of KX2-391, an oral inhibitor of Src kinase and tubulin polymerization, in men with bone-metastatic castration-resistant prostate cancer.

PURPOSE: 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). METHODS: We treated 31 patients with oral KX2-391 (40 mg twice-daily) until disease progression or unacceptable toxicity. The primary endpoint was 24-week progression-free survival (PFS); a 50 % success rate was pre-defined 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]. RESULTS: The trial closed early after accrual of 31 patients, due to a pre-specified 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 C max was 61 (range 16-129) ng/mL, and median AUC was 156 (35-348) ng h/mL. Common toxicities included hepatic derangements, myelosuppression, fatigue, nausea, and constipation. CONCLUSION: KX2-391 dosed at 40 mg twice-daily lacks antitumor activity in men with CRPC, but has modest effects on bone turnover markers. Because a C max of ≥142 ng/mL is required for tubulin polymerization inhibition (defined from preclinical studies), higher once-daily dosing will be used in future trials.

A phase I trial of KX2-391, a novel non-ATP competitive substrate-pocket- directed SRC inhibitor, in patients with advanced malignancies.

BACKGROUND: Src kinase is central to tumor cell proliferation, apoptosis, and metastasis. KX2-391 is a synthetic, orally bioavailable small molecule inhibitor of Src tyrosine kinase (TK) signaling and tubulin polymerization. This compound is distinct from other Src kinase inhibitors by targeting the peptide substrate rather than the ATP binding site; the binding site on hetero-dimeric tubulin is novel and distinct from the taxanes and other known tubulin inhibitors. METHODS: This multicenter Phase I trial utilized a 4 + 2 study design to determine the maximum tolerated dose (MTD), safety, and pharmacokinetics (PK) of KX2-391 in patients with refractory solid tumors. RESULTS: Forty-four (44) patients (18 M, 26 F; median age, 59) were enrolled in 9 dose cohorts. Dose-limiting toxicities, all reversible within 7 days, occurred in 7 patients and consisted of elevated AST (n = 4), ALT (n = 2), neutropenia (n = 1), thrombocytopenia (n = 1), failure to thrive (n = 1) and anorexia (n = 1). The MTD is 40 mg BID continuously. Eleven patients had stable disease for ≥ 4 months, including patients with ovarian, carcinoid, papillary thyroid, prostate, pancreas and head and neck cancer. Patients with prostate and pancreatic cancer also had significant biomarker decreases (PSA, 205 ng/mL to 39 ng/mL; CA19-9, 38,838 U/mL to 267 U/mL). The ovarian cancer patient has had stable disease > 12 months. KX2-391 was orally available, rapidly absorbed, and exposure was proportional to dose across the range investigated. CONCLUSIONS: KX2-391 has a favorable pharmacokinetic profile, is well tolerated, demonstrates preliminary evidence of biologic activity, and warrants further evaluation in Phase II trials.