Cabozantinib Shows Unprecedented Activity in Patients with Castration-resistant Prostate Cancer

Cabozantinib (XL184) reduces measurable soft-tissue lesions, shows resolution (complete or partial) or stabilization of bone metastases, decreases bone pain, and reduces narcotic use in a cohort of patients with metastatic castration-resistant prostate cancer (mCRPC), based on interim data from a phase II study reported at the Genitourinary (Prostate) Cancer Oral Abstract Session, held on Monday, June 6.

Maha Hussain, MD, of the University of Michigan Comprehensive Cancer Center, presented the data from a phase II randomized discontinuation study (Abstract 4516).

After a 12-week lead-in period when all patients were treated with 100 mg of once-daily oral cabozantinib, further treatment protocol was based on week-12 tumor staging.

Patients with partial or complete response continued on open-label cabozantinib. Patients with stable disease were randomly assigned in a blinded manner to receive cabozantinib or placebo. Patients with progressive disease discontinued cabozantinib. Although the study enrolled patients with multiple solid tumors, data were provided for the cohort of patients with mCRPC.

Cabozantinib is a tyrosine kinase inhibitor (TKI) with multiple activities. It blocks MET and vascular endothelial growth factor receptor (VEGFR). MET is implicated in tumor invasion and metastases, and the synergistic effects of MET and VEGFR promote tumor angiogenesis.

After 171 patients were enrolled in the study, randomization was suspended after 122 patients receiving cabozantinib showed clinical benefits. Of the 171 patients with mCRPC, lymph node, visceral, and bone metastases were seen in 87%, 37%, and 87% of patients, respectively. At baseline, 54% of patients had bone pain and 42% of patients were using narcotics for pain relief. Forty-three percent of patients had previously been treated with docetaxel.

At 12 weeks, disease-control rate, defined as partial response or stable disease, was seen in 68% of patients. For the 31 patients randomly assigned to either the placebo group (17 patients) or the cabozantinib group (14 patients), median progression-free survival (PFS) was significantly longer for patients on cabozantinib (21 weeks vs. 6 weeks for placebo; p = 0.0007).

Patients receiving cabozantinib were 87% less likely to experience tumor progression (hazard ratio: 0.13; log-rank p = 0.0007).

The most dramatic observations of cabozantinib were its effects on soft-tissue lesions and bone metastases. Seventy-four percent of patients showed evidence of measurable regression of soft-tissue lesions. Of 108 patients with evaluable bone scans, 21 patients (19%) showed complete resolution and 61 patients (56%) showed partial resolution of bone metastases. Only three patients (3%) showed progression of bone metastases.

Improvement in pain from baseline was seen in 56 patients (67%), and 31 patients (56%) decreased or discontinued narcotics for pain. Resolution of bone lesions was associated with corresponding decreases in levels of alkaline phosphatase and type I collagen C-telopeptides, which define osteoblast (bone-formation marker) and osteoclast (bone-resorption marker) activities.

Because docetaxel is the principal chemotherapy recommended for patients with mCRPC, PFS also was determined for the subsets of patients based on docetaxel pretreatment.

Ninety docetaxel-naïve patients and 64 patients previously treated with docetaxel experienced PFS of 29 weeks and 24 weeks, respectively.

Adverse events were moderate and manageable in most patients receiving cabozantinib and were similar to those seen in patients receiving TKIs. Grade 3 fatigue, palmar-plantar erythrodysesthesia syndrome, and hypertension were seen in 16%, 6%, and 6% of patients, respectively.

Discussant Daniel George, MD, of the Duke Cancer Institute, indicated that the side effects of cabozantinib were not remarkably different from sunitinib.

"Cabozantinib is not just another TKI. It shows unprecedented changes in bone scan and dramatic effects on bone pain." Dr. George questioned whether these endpoints will translate to prolonged survival.