Souza, Jonas A. de

e17580 Background: Cabozantinib is a small molecule multiple-receptor tyrosine kinase inhibitor that primarily targets MET and vascular endothelial growth factor receptor 2 (VEGFR2). We evaluated the effect of cabozantinib on BTM in RAIR DTC. We also assessed the response of BM to cabozantinib using 18F Sodium Fluoride (NaF) PET and radionuclide bone scans. Methods: This prospective analysis was performed as part of a multicenter International Thyroid Oncology Group phase II study of cabozantinibin patients with RAIR DTC who progressed on prior VEGFR-targeted therapy (NCT01811212). BTM [serum C-telopeptide (CTx), osteocalcin, N-telopeptide (NTx), procollagen type 1 (Pc1) and urinary NTx] and NaF PET or Tc-MDP scans were done at baseline and after 8 weeks of therapy. A paired t-test was used between baseline and week 8 BTM. Results: Primary study results were presented elsewhere (Shah, et al. 2015, International Thyroid Congress) showing a response rate of 36% with cabozantinib. Of 25 patients (pts) enrolled, 21 had BM, 10 of whom were on bone-modifying agents (BMA) prior to week 8. Only 2 pts had elevated BTM at baseline. In all pts who had paired BTM, there was a statistically significant decline in all BTM except NTx by week 8 (table). Pts not on BMA also had a significant decline in CTx (p 0.0003), urine NTx (p 0.0300), and Pc1 (p 0.0030) compared to baseline. 19 pts were evaluable for overall response and there was no correlation between BTM and response rate. 6/21 patients underwent paired NaF PET scans and 4/6 had minor reduction in lesion uptake at 8 weeks. Conclusions: Interestingly, most of our patients had normal baseline BTM despite BM. Cabozantinib was associated with a strikingly significant reduction in all BTM (even when BTM were within normal limits at baseline) except NTx by week 8. Minor decreases in bone lesion uptake was noted on NaF PET in 4/6 evaluable patients. Clinical trial information: NCT01811212. [Table: see text]

Purpose Sorafenib and lenvatinib are oral multikinase inhibitors targeting vascular endothelial growth factor receptor (VEGFR) and approved for radioiodine (RAI)-refractory differentiated thyroid cancer (DTC). However, there are no approved second- or third-line therapies. MET is implicated in resistance to VEGFR inhibitors. Cabozantinib is an oral multikinase inhibitor targeting MET in addition to VEGFR and is approved for medullary thyroid cancer. In a phase I study of cabozantinib, five of eight patients with DTC previously treated with a VEGFR-targeted therapy had an objective response to cabozantinib. Patients and Methods Patients with RAI-refractory disease with Response Evaluation Criteria in Solid Tumor (RECIST) measurable disease and evidence of progression on prior VEGFR-targeted therapy were enrolled in this single-arm phase II study. The cabozantinib starting dose was 60 mg/day orally but could be escalated to 80 mg if the patient did not experience a response. Patients underwent tumor assessment according to RECIST v1.1 every 8 weeks. In this study, if at least five of 25 response-evaluable patients had an objective response, cabozantinib would be considered a promising agent in this patient population. Results Twenty-five patients were enrolled. The median age was 64 years, and 64% of patients were men. Twenty-one patients had received only one prior VEGFR-targeted therapy (sorafenib, pazopanib, or cediranib), and four patients had received two such therapies. The most common treatment-related adverse events were fatigue, weight loss, diarrhea, palmar-plantar erythrodysesthesia, and hypertension. One drug-related death was noted. Of the 25 patients, 10 (40%) had a partial response, 13 (52%) had stable disease, and two (8%) had nonevaluable disease. The median progression-free survival and overall survival were 12.7 months and 34.7 months, respectively. Conclusion Cabozantinib demonstrated clinically significant, durable objective response activity in patients with RAI-refractory DTC who experienced disease progression while taking prior VEGFR-targeted therapy.