Phase 1, Open-Label, Dose Escalation Study of I-131-CLR1404 in Patients with Relapsed or Refractory Multiple Myeloma (RRMM)

Background: I-131-CLR1404 is a novel radioiodinated therapeutic that exploits the selective uptake and retention of phospholipid ethers (PLEs) by malignant cells. Based on initial preclinical and clinical experience and the exquisite radiosensitivity of MM, I-131-CLR1404 is being examined in RRMM in an open-label, dose escalation Phase 1 trial (NCT02278315) evaluating up to 5 cohorts and a maximum of 37.5 mCi/m2. Methods: The primary objective is to determine safety and tolerability of I-131-CLR1404 with and without dexamethasone (dex) in RRMM. Secondary objectives are to determine the recommended Phase 2 dose (RP2D) and therapeutic activity in RRMM. Eligibility criteria include progressive, relapsed/refractory MM, and at least one previous exposure to proteasome inhibitor (PI) and immunomodulatory (IMiD) drugs. Prior autologous stem cell transplant (ASCT) and external beam radiation therapy are allowed (≤ 20% of total marrow irradiated). There is no limit to the number of prior therapies. I-131-CLR1404 is administered intravenously as a single 30 minute infusion on day 1 with 40 mg dex orally weekly for 12 weeks. Dose-escalation uses a minimally modified, standard 3+3 schema with dose-limiting toxicities (DLTs) assessed through day 85 post-infusion. Adverse events (AEs) are graded by NCI-CTCAE v4.03; responses are assessed by the International Myeloma Working Group Uniform Response Criteria. Results: Ten patients have been enrolled in the study with data presented as of 30 Jun 2016 for 8 patients. Accrual is ongoing in the additional cohorts. Five patients were enrolled to cohort 1 (12.5 mCi/m2 + dex); 4 were evaluable and 1 patient was taken off study for rapid disease progression. Five patients have been enrolled to cohort 2 (18.75 mCi/m2 + dex), with data presented on 3 enrolled prior to data cutoff. The median age was 68.5 (range 55-85) and included 5 males and 3 females. All patients were standard risk (karyotyping) and median bone marrow plasma cell involvement was 25% (range 4-50%). Median number of prior therapies was 4 (range 2-12) (Table 1). Four of 8 patients previously underwent ASCT and 25% were previously treated with radiation therapy. Hematologic AEs (grade 3+) and non-hematologic AEs (grade 2+) potentially related to study drug are shown in Table 1. No difference in the toxicity profile was noted for patients with previous radiation therapy. Two serious adverse events (SAEs) have been reported, both in patients treated with 18.75 mCi/m2 I-131-CLR1404. One patient was hospitalized for a seizure (Day 62), assessed as unrelated to study drug and another patient was hospitalized for a grade 1 upper GI bleed in the setting of grade 4 thrombocytopenia (Day 23). This is believed to be the result of rapidly progressive disease, however relationship to I-131-CLR1404 could not be ruled out. Stable disease was achieved in 100% of evaluable patients (n=7). One subject in cohort 1 demonstrated a > 30% reduction in serum m-protein. Figure 1 provides a summary of percent change in serum free light chain assay (FLC) from baseline. Three patients had a reduction in FLC > 50%, 2 in cohort 1 and 1, to date, in cohort 2. Three patients had a stable FLC response through at least 22 days following treatment and 1 subject was unresponsive with regard to FLC. Median follow-up is 6.8 months (range 1.2-14.2). Patients did not receive subsequent systemic therapy for a median of 3.5 months (range 0.6-4.7). Furthermore, 1 subject in cohort 2 maintains stable disease and has not initiated subsequent treatment. Median progression free survival (PFS) is 3 months and included 4 patients with stable disease at the end of the 85 day study follow-up. In patients with progression after I-131-CLR1404, treatment included daratumumab (n=4), ASCT (n=1), and bortezomib, lenalidomide, and dex (VRD) (n=1). Conclusions: I-131-CLR1404 represents a unique, first in class targeted radiotherapeutic for MM. Preliminary data from this trial show an acceptable and expected safety profile with an efficacy signal in the initial 2 of a potential 5 cohorts. All evaluable patients, despite extensive previous therapy and refractoriness to new therapies, achieved disease stabilization. The study is ongoing to determine the RP2D and long term efficacy. Disclosures Ailawadhi: Pharmacyclics: Consultancy; Novartis: Consultancy; Amgen Inc: Consultancy; Takeda Oncology: Consultancy. Oliver:Cellectar Biosciences: Employment.