MoAb Used for In Vivo Purging of Stem Cells Before Transplantation

MIAMI BEACH—Bone marrow and peripheral blood stem cell transplants after high-dose chemotherapy are becoming more widely used, but there is no standard method of purging graft material to remove tumor cells prior to transplant.

In vitro approaches have included chemical methods, CD34 selection, and a cocktail of monoclonal antibodies used to treat graft material before it is reinfused.

Two studies presented at ASH suggest that it may be possible to do “in vivo” purging in patients with non-Hodgkin’s lymphoma using the anti-CD20 monoclonal antibody rituximab (Rituxan).

Ian W. Flinn, MD, reported that a rituximab-based combination “is a well-tolerated regimen that successfully depletes stem cell grafts of CD20+ cells, provides rapid engraftment, and is associated with little added toxicity.”

Dr. Flinn and his colleagues at Johns Hopkins studied rituximab for in vivo purging and as post-transplant adjuvant immunotherapy in NHL. Their goals were to reduce contamination of the stem cell graft with lymphoma and to reduce residual disease remaining in the patient after high-dose therapy—two causes of relapse after autologous transplant.

“Our problem was that we had reached the maximum tolerance of our preparative regimens. We were looking for some sort of adjuvant as a nontoxic way of adding to the protocol,” he said.

In this protocol, patients first receive rituximab; then, to mobilize stem cells, cyclophosphamide and G-CSF are given 3 days after rituximab. Stem cells are collected, the patient receives a preparative regimen, and the stem cells are reinfused. Once the patient has recovery of platelets and neutrophils, another rituximab dose is given, Dr. Flinn said.

No CD20+ Cells Detected

In this preliminary report, stem cells were successfully mobilized in 21 of 22 patients. Fifteen patients required only one apheresis. No CD20+ cells were detectable by flow cytometry in any of the grafts. “However,” he said, “quantitative PCR will need to be done to further evalute purging.”

Rituximab was very well tolerated, with a very low rate of infusion-related side effects, he said. “That may be because our candidates for transplant had only minimal residual disease.” Toxicities include one patient who developed pancytopenia and subsequently died; two with zoster; and one with thrombocytopenia who responded rapidly to steroids.

“In our next trial, we will be giving four doses of rituximab after transplant rather than one,” Dr. Flinn said. “We will also be using GM-CSF as adjuvant therapy post-transplant. GM-CSF may increase antibody-dependent cellular cytotoxicity.”

R. K. Gupta, MD, and colleagues at St. Bartholomew’s Hospital, London, reported that PCR analysis for the t(14;18) translocation in patients with recurrent follicular lymphoma showed similar purging following rituximab. This translocation can be found in up to 80% of patients with follicular lymphoma and is used as a surrogate marker, particularly in minimal residual disease.

PCR Results

Dr. Gupta monitored minimal residual disease in 58 patients treated with rituximab in a large UK trial. Prior to rituximab treatment, 31 patients (53%) were PCR positive for the t(14:18) marker in lymph node tissue (13 patients) and/or bone marrow (12 patients) and/or peripheral blood (20).

Follow-up bone marrow and blood samples (obtained a month after the last rituximab infusion) were available in 28 of these patients. Overall, 17 (61%) became PCR negative after treatment. Of the 13 nonresponders, 7 (54%) became PCR negative. Partial remissions occurred in 15 patients; 10 of 15 (67%) who became PCR negative had a PR vs 5 of 11 (45%) who remained PCR positive.

“This suggests that rituximab is very effective in decreasing or eliminating CD20+ cells, including follicular lymphoma cells, in marrow or blood irrespective of clinical response,” he said. “Although obtaining a PCR-negative state does not reflect clinical outcome, rituximab may be useful as a means of in vivo purging.”