R-DHAP Is Effective in Fludarabine-Refractory CLL, Possibly Via Upregulation of Pro-Apoptotic P73.

Abstract 3449

Poster Board III-337

Allogeneic stem cell transplantation (alloSCT) has the potential to cure high risk CLL patients including those with fludarabine-refractory disease with or without a deletion of 17p. Treatment-related mortality (TRM) of this procedure is significantly reduced by the use of reduced-intensity conditioning regimens (RIC). Risk factors for relapse after alloSCT are refractory or bulky disease.

In an ongoing prospective Dutch/ Belgian HOVON trial we study the efficacy of remission-induction with the R-DHAP regimen (rituximab, cytarabine, cisplatinum and dexamethasone) prior to alloSCT in CLL patients with fludarabine refractory disease. During the initiation phase ten fludarabine refractory patients have been treated according to this protocol. The table shows their disease characteristics and response to R-DHAP.

Eight of ten patients had a response: one CR and seven PR, whereas two had SD. In the patient who achieved a CR, four-color flowcytometry confirmed the absence of minimal residual disease (MRD). One of the two patients with del 17p and both patients with del 11q had a response. Six of the eight patients with bulky disease responded. One patient developed tumor lysis syndrome. One patient, who had been pretreated with alemtuzumab (no 8), developed an opportunistic infection (Aspergillus pneumonia). Most responding patients subsequently underwent alloSCT.

The observed response rate in these heavily pre-treated and chemo-refractory patients is remarkable. As chemorefractory disease is highly associated with a dysfunctional p53 response, we hypothesized that effects of this regimen could be independent of p53-function.

Therefore the molecular basis of apoptosis induction was studied more in depth in one patient with del 17p and proven p53 dysfunction. Within weeks after the first cycle of R-DHAP peripheral blood lymphocyte counts decreased from 78 × 10⁹/L to 3.3 × 10⁹/L. Simultaneous analysis of RNA-expression levels of 30 apoptosis regulating genes by RT-MLPA in samples taken before and 24 and 48 hours after start of R-DHAP revealed significant upregulation of the pro-apoptotic BH3-only molecule Puma. Recent data indicate that Puma is not only a p53 response gene, but can also be regulated by the p53-family member p73. Indeed, detectable p73 protein levels were found already after 24 hours of treatment in vivo. At the functional level, we found that the pretreatment fludarabine resistance in vitro was abrogated after 48 hours of treatment in vivo.

Our data indicate that the R-DHAP regimen has activity in fludarabine-refractory CLL patients, even in those with cytogenetic changes affecting the p53-response. Furthermore, in our preliminary in vitro studies we could show upregulation of the pro-apoptotic protein p73 and concomitant abrogation of fludarabine-resistance. This pathway should be further explored and may provide means to overcome drug-resistance in CLL.