Meredith, Ruby F.

Abstract Rationale Escalation of the Zevalin dose is limited by hematologic toxicity associated with marrow B-cells ± tumor cells. Rituxan treatment before Zevalin may reverse marrow infiltration with malignant and normal B-cells allowing an increase in the dosage of Zevalin with enhanced radiation delivery to lymphoma tumor sites. Since NHL is radiation sensitive, a substantial increase in radiation dose delivered should increase the ORR and CR rates. Design Low-grade follicular/transformed NHL patients receive 4 weekly dose of Rituxan (375mg/m2) before the corresponding Zevalin dose. Bilateral bone marrow biopsies/aspirates as well as biodistribution and dosimetry studies using 111In-2B8 were obtained before and after Rituxan. The trial was schedule to escalate the Zevalin dose (from 0.4 to 0.7 mCi/Kg). After completion of the 0.4 mCi/kg cohort an additional cohort of Rituxan sensitive patients using 0.3 mCi/kg was completed. Safety, pharmacokinetics, dosimetry and preliminary responses are presented. Results 5 pts were enrolled in the 0.4mCi/kg cohort and 6 pts in the 0.3 mCi/kg cohort. In the 10 Rituxan sensitive patients the plasma T ½ and the AUC of 111In-2B8 increased after the administration of Rituxan, with a reduction in the plasma clearance (T½ from 45 ± 4.5 hours to 54 ± 1.9 hours, AUC from 60 ± 10 hrs/μCi/ml to 85 ± 14 hrs/μCi/ml, and the clearance from 1.4 ± 0.3 ml/hrs/kg to 0.9 ± 0.2 ml/hrs/kg). Dosimetry showed no differences in the radiation-absorbed dose for major normal organs (whole body, liver, kidneys) except for the spleen in which a reduction was observed in 7 out of 10 patients. The radiation-absorbed dose to the marrow increased after Rituxan; from 3.6 rads/mCi to 4.28 rads/mCi. In the 0.4 mCi/kg cohort 3 out of 4 evaluable patients for response had a complete response, and 1 patient had progression documented at initial evaluation six weeks post-Zevalin (patient refractory to previous Rituxan). No non-hematological toxicity was observed. One patient had a grade 4 thrombocytopenia, 3 patients had grade 3 thrombocytopenia, and 4 patients had grade 4 WBC and ANC. All patients recovered within two weeks after the nadir. Two out of 6 patients treated in the 0.3 mCi/kg cohort had complete response by physical evaluation, and 4 had partial response; no early progressions were seen in this cohort. No non-hematological toxicity was observed. Only one patient had a grade 4 WBC and ANC, and 2 patients had a grade 3; 3 patients had grade 3 thrombocytopenia; all patients recovered. Conclusions The administration of high doses of Rituxan before Zevalin prolongs the circulation time of the radiolabeled antibody and increases the bone marrow exposure to the radioisotope. Additionally, the increased amount of the radiolabeled antibody available after Rituxan therapy with the prolonged circulation time may explain the unusual high complete response rate observed in this small number of patients despite extensive chemotherapy failures. Although this is very provocative data, the number of patients is very limited to generate a strong conclusion. The trial continues at this time.

Abstract Abstract 1648 Introduction Radioimmunotherapy capitalizes on the radiosensitivity of non-Hodgkin lymphoma (NHL) and the targeted nature of monoclonal antibody therapy. In an attempt to reverse bone marrow infiltration with B-cells and, hence, optimize the biodistribution and tumor dosimetry of 90Y-ibritumomab tiuxetan, we conducted a multicenter, dose escalation, open label study to determine maximum tolerated dose (MTD), safety, and efficacy of a single course of 90Y-ibritumomab tiuxetan after four weekly doses of rituximab in relapsed or refractory low-grade or follicular or transformed CD20+ B-cell NHL. Methods A course of therapy included an initial dose of rituximab 250 mg/m2 plus 5 mCi of 111In- ibritumomab tiuxetan followed by three weekly doses of rituximab at 375 mg/m2 followed two weeks later by rituximab 250 mg/m2 plus 5 mCi of 111In-ibritumomab tiuxetan. Gamma camera imaging for dose estimates was conducted on days 1 and 36 following infusion of 111In- ibritumomab tiuxetan. If dose estimates were in the safe defined limits, and bone marrow on day 29 had less than 25% involvement by NHL, patients received on day 43 250 mg/m2 of rituximab plus a dose of 90Y-ibritumomab tiuxetan. Patients were scheduled to receive 90Y- ibritumomab tiuxetan in one of the dose escalation cohorts (0.4, 0.5, 0.6, 0.7 mCi/kg). Eligible patients were required to have histologically confirmed, relapsed or refractory low-grade or follicular or transformed B-cell NHL with demonstrable monoclonal CD20+ B-cell population in the lymph nodes or bone marrow. Patients with impaired bone marrow reserve, as indicated by prior myeloablative therapies with stem cell rescue, hypocellular marrow, history of failed stem cell collection or external beam radiation therapy to >25% of active bone marrow, and marked reduction in bone marrow precursors of one or more cell lines, were excluded. Results Cohort 1 (0.4 mCi/kg) enrolled 5 patients. Dose escalation was held after significant hematologic toxicity was observed in those patients (1 patient developed grade 4 and 3 grade 3 reversible thrombocytopenia, 4 patients developed grade 4 reversible leukopenia and neutropenia, and 1 patient developed grade 4 febrile neutropenia). One patient in this cohort did not receive the study drug due to the toxicity observed in the previous patients. The protocol was amended and 6 patients were enrolled at a lower dose cohort (0.3 mCi/kg); 1 patient experienced grade 4 neutropenia, whereas grade 3 leukopenia, neutropenia, and thrombocytopenia were seen in 2, 3, and 3 patients respectively. As the 0.3 mCi/kg dose was well tolerated, the 0.4 mCi/kg cohort was expanded to enroll 6 additional patients. In the expansion cohort, Grade 4 leukopenia and neutropenia were seen in 1 and 2 patients respectively; also, grade 3 leukopenia, neutropenia, and thrombocytopenia were seen in 3, 4, and 4 patients, respectively. All hematologic toxicities were reversible and no non-hematologic toxicities were seen with the exception of grade 1 fatigue in 2 patients. Among those who developed hematologic toxicities, the median duration of at least grade 3 leukopenia, neutropenia, and thrombocytopenia were 12 (range, 7–37), 10 (7–30), and 12 (3–44) days, respectively. Six patients had bone marrow involvement by NHL at registration ranging from <5% to 20%. Following 4 weekly doses of rituximab, marrow involvement decreased in all patients with complete clearance in 3 of them. In an intention-to-treat analysis, complete remission, partial remission, stable and progressive disease was achieved in 6 (35%), 8 (47%), 2 (12%), and 1(6%) patients, respectively (fig 1). With a median follow up of 31.7 months (range, 4.3 – 92.6 months), the median progression free survival (fig 2) and time to next treatment (fig 3) were 12.3 and 10.9 months, respectively. The median overall survival has not been reached. Conclusion The MTD of 90Y-ibritumomab tiuxetan administered to patients who have NHL marrow infiltration <25% following 4 weekly doses of rituximab was 0.4 mCi/kg. A 4 weekly course of rituximab decreased NHL marrow infiltration in all patients with marrow involvement and led to complete clearance in 50% of them. Although this regimen was associated with a high response rate, the hematologic toxicity was significant. Disclosures: No relevant conflicts of interest to declare.