Impaired Clonogenic Capacity Contributes to Bone Marrow Hypoplasia and Late Hematological Recovery after Zevalin in the Low-Grade NHL Patients.

Chemoimmunotherapy with Zevalin (anty CD20,⁹⁰Y) is a new therapeutic approach in the resistant or relapsed low grade malignant lymphoma patients. The most important side effect observed in the up to date studies is related to transient peripheral blood cytopenias.

In an attempt to evaluate the involved pathogenetic mechanisms of the above findings, the blood and bone marrow samples from 8 low grade NHL patients were prospectively tested for the peripheral blood count, the clonogenic capacity and the histopathology abnormalities (trephine biopsies) before and every 14 days after Zevalin administration until recovery. In the majority evaluated patients the mild thrombocytopenia (30-55000/μl) was observed 4 weeks after Zevalin therapy. Bone marrow mononuclear cells were cultured in methylcellulose assay using combination of recombinant CSF (IL-3, IL-6, stem cell factor, erythropoietin; Methocult, StemCell, Canada), at a concentration of 1x10^5 cells, in 35 mm gridded Petri dish and incubated at 37°C, in a humidified atmosphere of 5% CO₂. The hematopoietic colonies (CFU-GM, BFU-E and CFU-GEMM) were evaluated on 14th d. of the culture. The in vitro CFU-Meg formation in the serum-free collagen-based semisolid culture (Megacult, StemCell, Canada) were also studied, stained after 12 days with a monoclonal antibody against CD 41.

In the results of our studies the significant inhibition of colony formation was found already in 7th day for CFU-GM and BFU-E and starting from 14th day after Zevalin administration for CFU-Meg. In the subsequent cultures evaluated during 4–8 weeks after Zevalin therapy the three lineages hematopoietic clonogenic capacity returned to the pre-radiotherapy values. In the repeated trephine biopsis several morphological changes were observed: oedema of stromal cells, the relative increase of erythropoietic components with inverted proportion of granulo- to erythropoiesis.

In conclusion, chemoimmunotherapy with Zevalin resulted in the reversible peripheral cytopenia parallely with transient damage to bone marrow hematopoiesis and in vitro clonogenic capacity.