Abstract

Abstract 2747

Background:

Single-agent immunotherapy with rituximab is a viable treatment option for low risk FL, with limited toxicity and a long duration of response in some patient subsets. We have previously shown that high expression of FcγRIIB promotes rituximab internalisation on various B cell targets, including FL (Blood 2011 118:2530–2540), something not seen with type II anti-CD20 antibodies. The SAKK 35/98 trial examined rituximab monotherapy in FL and now has long-term follow-up data of almost 10 years (JCO 2010 28:4480–4484). We analysed diagnostic tumour samples from this trial to determine the relationship of FcγRIIB expression to responses and clinical outcomes after rituximab treatment in FL.

Methods:

202 patients (pts) with newly diagnosed or relapsed FL received induction treatment with rituximab 375 mg/m2 weekly for 4 weeks. Pts with stable or responding disease at week 12 were randomized into 2 groups: no further treatment or prolonged treatment with single infusions of rituximab 375 mg/m2 at weeks 12, 20, 28 and 36. Archived tissue samples from 135 evaluable pts were stained using an anti-human FcγRIIB antibody (clone EP888Y, Abcam) at a dilution of 1:3000 on a Dako autostainer. The samples were pretreated with the Dako EnVisionFLEX target retrieval solution high pH and detection using the Dako AS-Link 48 with Dako EnVision flex plus detection kit. Positive samples were graded into negative/low intensity staining (n=120) versus medium/high (n=13) by an expert lymphoma histopathologist blinded to the clinical outcomes. Data from 2 slides and response at week 12 data for 4 pts were unavailable (1 of whom also has missing slide data), resulting in 130 pts available for analysis. Failure-free survival (FFS) was defined as time from registration until failure to achieve complete/partial response at week 12, progression, relapse, a second cancer or death from any cause. Objective response rate (ORR) was associated with intensity staining levels using Fisher's exact test. All time-to-event endpoints were evaluated using the Kaplan-Meier method; groups were compared using the log-rank test. The hazard ratio (HR) was assessed using Cox proportional hazards models.

Results:

Registered and randomised pts had very similar baseline characteristics; previously untreated pts had slightly more favourable characteristics but were balanced between the 2 treatment arms. Pts expressing medium/high levels of FcγRIIB were less likely to respond to rituximab by week 12 (ORR 58.1% vs 23.1%, Fisher's exact test, p=0. 02), a finding independent of prior therapy. For FFS, there was a statistically significant difference (p=0.001; HR=0.42; 95% confidence interval (C.I.): 0.23–0.77) between the negative/low staining group (median: 21.4 months; 95% C.I.: 7.0–34.2) and the medium/high staining group (median: 7.0 months; 95% C.I.: Not calculable). The interaction between staining levels and randomised treatment groups for FFS was not statistically significant. There was a non-significant trend towards better overall survival in the low/negative group (median: 140.0 vs 50.0 months; p=0.15; HR=0.57; 95% C.I.: 0.27–1.23); however the event rate was lower (36.8% vs 61.5%).

Conclusion:

Elevated FcγRIIB expression level is associated with poor response to rituximab in pts with FL. This group may show better results with non-internalising type II antibodies, a hypothesis for validation in future prospective clinical trials.

Disclosures:

Ghielmini:Roche: Honoraria, Speakers Bureau. Johnson:Roche: Honoraria.

Author notes

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Asterisk with author names denotes non-ASH members.