Biochemical Tumor Marker Status and Its Role in Treatment Response in Patients Who Received High-Specific-Activity I-131 MIBG in Advanced Pheochromocytoma and Paraganglioma (PPGL): Results From a Pivotal Phase 2 Clinical Trial

Background: High-specific-activity iodine-131 meta-iodobenzylguanidine (HSA I-131 MIBG; AZEDRA®) has been approved for the treatment of adult and pediatric patients (pts) 12 years and older with iobenguane scan positive, unresectable, locally advanced or metastatic PPGL who require systemic anticancer therapy. We have previously presented data showing improved biomarker responses in pts treated with HSA I-131 MIBG. Here we report the impact of biomarker status on the study primary endpoint and objective tumor response. Methods: Pts with iobenguane-avid PPGL who were ineligible for surgery, failed prior therapy or not candidates for chemotherapy, and on a stable antihypertensive medication regimen were treated. Pts received up to two therapeutic doses, each at ~18.5 GBq (or 296 MBq/kg for pts ≤62.5 kg), administered ~90 days apart. Biomarkers were analyzed at baseline and over a 12-month efficacy period. Confirmed biochemical responses (at least ≥ 50% decrease in abnormal tumor marker value for all hypersecreted biomarkers) required subsequent responses to be identical to or better compared with the previous assessment. The primary endpoint was clinical benefit, defined as the proportion of pts with at least 50% reduction of all antihypertensive medication(s) for ≥6 months beginning during the efficacy period. The secondary endpoint, confirmed objective tumor response by RECIST, was also evaluated. Results: 68 pts received at least one therapeutic dose of HSA I-131 MIBG. For all pts with hypersecretory tumors (with a baseline biochemical marker level of ≥1.5× ULN) (n=60), a comparison of biomarker response with antihypertensive therapy yielded a correlation coefficient of 0.35 (P = 0.006; Fisher exact P = 0.012). For pts with norepinephrine only-hypersecreting tumors (n=31), a correlation coefficient of 0.47 (P = 0.008; Fisher exact P = 0.015) was observed. The overall biomarker response also correlated with objective tumor response (n=55) yielding a correlation coefficient of 0.36 (P = 0.007; Fisher exact P = 0.012) for all pts with hypersecreted biomarkers. Pts who were not biochemical hypersecretors for any biomarker (n=6) had only one responder for the primary endpoint and no objective tumor responses. Conclusions: The biomarker data from this study establish a moderate but statistically significant correlation between biomarker response following treatment with HSA I-131 MIBG and objective tumor response and durable reduction of antihypertensive therapy. This correlation was improved with norepinephrine only-hypersecreting tumors in pts with unresectable, locally advanced or metastatic PPGL.