We next evaluated the ADCC potential of GBR 401, in comparison with RTX, the standard mAb for lymphoma therapy and herein used as a positive control and reference. When we tested the ADCC activity on cell lines where CD19 and CD20 were highly expressed (Raji, Ramos and SU-DHL-6, Table
1), GBR 401 and RTX showed a comparable maximum killing efficacy (Figure
2B). However, Namalwa cells that express low levels of CD20 (Table
1) responded solely to GBR 401. Despite similar maximum killing efficacies, and substantially higher levels of CD20 compared to CD19 (Table
1), GBR 401 demonstrated an overall 6-10 fold lower EC50 compared to RTX on various malignant B cells that include Raji, Ramos and SU-DHL-6 cells indicating a better ADCC activity at low doses (Table
1 and Figure
2B). Of interest, we also investigated whether GBR 401 was able to kill primary malignant cells. To this end, peripheral blood mononuclear cells (PBMC) from patients diagnosed with CLL and other types of lymphomas (mantle cell lymphoma, Waldenström lymphoma, hairy cell leukemia-variant and post-transplant lymphoproliferative disorder) were treated with increasing concentrations of GBR 401 or RTX in the presence of NK cells purified from two healthy donors. As shown in Figure
2C, GBR 401 showed a strikingly potent
in vitro ADCC activity against B-CLL cells. Indeed, it decreased by 500 fold (
P < .001) the mean EC
50 values (ranging between 0.056 and 15.2 ng/mL) compared with RTX (15-5000 ng/mL). The maximum killing efficacy of GBR 401 was also significantly superior (
P < .001) to that of RTX (Figure
2C). With primary lymphoma samples, GBR 401 reduced EC
50 values (
P < .01, 0.05-93 ng/mL) compared with RTX (5-580 ng/mL) (Figure
2D), even though there was no difference in their maximum killing efficacy. These results can partially be explained by the relatively low expression of CD20 in our CLL samples (Table
1 and Additional file
1: Figure S1), which is in agreement with literature data [
16,
25]. However, CD19 expressions in CLL samples are very low when compared with our cell lines and are not vastly different from those of CD20 (Table
1 and Additional file
1: Figure S1) [
25]. Similarly, CD20 levels on lymphoma cells have been reported to be higher than CD19 levels [
25]. These observations indicate that the better activity of GBR 401 on CLL and lymphoma samples cannot be solely explained by a bias in CD19 levels but rather by a better cytotoxic activity.
Taken together, these data demonstrate that GBR 401 displays potent ADCC activity against various cell lines and primary CLL and lymphoma samples. Interestingly, GBR 401 shows far superior ADCC potential over RTX in settings where CD20 expression is reduced as is seen in some CLL samples and in Namalwa cells. This indicates that GBR 401 could be an interesting alternative therapeutic agent in such conditions where RTX is poorly effective.