A Preliminary Report of a Phase I Study of Zevalin® Using a Modified Treatment Regimen for Relapsed or Refractory CD20+ B-Cell Follicular or Transformed Non-Hodgkin’s Lymphoma (NHL).

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.