Biomarkers and Bone Imaging Dynamics associated with Clinical outcomes of Oral Cabozantinib Therapy in Metastatic Castrate Resistant Prostate Cancer

Background: Cabozantinib is a multitargeted tyrosine kinase inhibitor that demonstrated remarkable responses on bone scan in metastatic prostate cancer. Randomized trials failed to demonstrate statistically significant overall survival (OS). We studied the dynamics of biomarker changes with imaging and biopsies pretherapy and posttherapy to explore factors that are likely to be predictive of efficacy with cabozantinib. Methods: Eligibility included patients with metastatic castrate-resistant prostate cancer with normal organ function and performance status 0–2. Cabozantinib 60 mg orally was administered daily. Pretherapy and 2 weeks post, 99m Tc-labeled bone scans, positron emission tomography with 18 F-sodium fluoride (NaF-PET) and 18 F-(1-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) thymine (FMAU PET) scans were conducted. Pretherapy and posttherapy tumor biopsies were conducted, and serum and urine bone markers were measured. Results: Twenty evaluable patients were treated. Eight patients had a PSA decline, of which 2 had a decline of ≥50%. Median progression-free survival (PFS) and OS were 4.1 and 11.2 months, respectively, and 3 patients were on therapy for 8, 10, and 13 months. The NaF-PET demonstrated a median decline in SUVmax of −56% (range, −85 to −5%, n = 11) and −41% (range, −60 to −25%, n = 9) for patients who were clinically stable and remained on therapy for ≥4 or Conclusions: Early changes in imaging and tissue or serum/urine biomarkers did not demonstrate utility in predicting clinical benefit with cabozantinib therapy. Clin Cancer Res; 1–11. ©2018 AACR.