Potent Preclinical Activity of Ponatinib in Germinal Center B-Cell-like Diffuse Large B-Cell Lymphoma (GCB-DLBCL) Models

Introduction

Diffuse large B-cell lymphoma (DLBCL), the most common type of Non-Hodgkin lymphoma (NHL), comprises 2 major molecular subtypes: germinal center B-cell-like (GCB) and activated B cell-like (ABC). Although standard therapy (rituximab+ chemotherapy [R-CHOP]) is effective in most patients (pts), a significant proportion do not achieve durable remissions. Treatment of relapsed and refractory DLBCL pts with targeted therapy, such as the BTK inhibitor ibrutinib, has shown some promise; however, responses are mostly restricted to the ABC subtype. Treatment options for pts with relapsed/refractory GCB, outside of stem cell transplantation, are especially limited.

Ponatinib is a potent pan-BCR-ABL inhibitor approved for pts with refractory or T315I+ chronic myeloid leukemia or Ph+ acute lymphoblastic leukemia. Initial characterization of the in vitro kinase activity of ponatinib demonstrated substantial activity against a number of additional oncogenic kinases, including KIT, RET, FLT3, and members of the FGFR, PDGFR, and SRC families. To obtain a broad, unbiased, assessment of the anti-proliferative effects of ponatinib, we screened a panel of 246 human tumor cell lines.  Based on the novel finding that a GCB-DLBCL cell line was amongst those inhibited most potently by ponatinib, we conducted studies to further characterize the activity of ponatinib in NHL, and GCB-DLBCL in particular.

Results

A broad cell-based screen identified a small subset of cell lines (18/246; 7%) whose growth was potently inhibited by ponatinib (GI₅₀<42 nM). A majority of these lines express activated variants of previously validated targets of ponatinib: ABL (N=5, GI₅₀ <0.3 nM), FLT3 (N=1, GI₅₀ 1 nM), FGFR2 (N=2, GI₅₀s 5-29 nM), and PDGFRα (N=1, 14 nM).  In addition, ponatinib potently inhibited growth of the GCB-DLBCL cell line DoHH2 (GI₅₀ 8 nM).  The cellular activity of ponatinib was next examined in a larger set of NHL cell lines enriched for the GCB subtype (Table 1).  Ponatinib only exhibited modest activity (GI₅₀ 46-119 nM) against 2 mantle cell lymphoma (MCL) lines, but potently inhibited growth (GI₅₀≤10 nM) of the one Burkitt’s lymphoma (BL) line tested (Daudi).  Most notably, ponatinib also potently inhibited growth of 5/9 GCB cell lines.  In contrast, none of the GCB lines showed sensitivity to ibrutinib (GI₅₀s >100 nM). Finally, we evaluated the in vivo potency of ponatinib in mice implanted with the GCB cell lines exhibiting the greatest (SU-DHL-4) and weakest (SU-DHL-10) in vitro sensitivity to ponatinib, using dosing regimens previously shown to be active in BCR-ABL models predictive of efficacy in patients. Once-daily oral administration of ponatinib resulted in a dose-dependent inhibition of SU-DHL-4 tumor growth, with 10 mg/kg inducing 78% tumor regression, and 30 mg/kg rapidly inducing complete regression that was maintained in all mice for an additional 2 weeks after ponatinib dosing was stopped.  In contrast, ponatinib had much more modest effects on SU-DHL-10 tumors with 30 mg/kg only inhibiting tumor growth by 39%.

Conclusion

Ponatinib has promising in vitro and in vivo activity against a substantial subset of GCB-DLBCL models tested, with potency similar to that observed in BCR-ABL models.  These results provide support for evaluating ponatinib in GCB-DLBCL pts who have failed prior therapy. Studies to further characterize the molecular basis for the activity of ponatinib in NHL are ongoing.

Table 1: In vitro drug activity in 12 NHL cell lines           

Cell line	Type	Ponatinib GI50 (nM)	Ibrutinib GI50 (nM)
SU-DHL-4	GCB DLBCL	1.3	313
DoHH2	GCB DLBCL	2.5	114
Pfeiffer	GCB DLBCL	6	2,074
SU-DHL-6	GCB DLBCL	9.8	1,041
WSU-NHL	GCB DLBCL	10	1,672
Farage	GCB DLBCL	51	1,409
U-2932	GCB DLBCL	79	>10,000
RL	GCB DLBCL	212	6,939
SU-DHL-10	GCB DLBCL	238	2,827
Daudi	BL	2.9	4,319
Mino	MCL	46	>10,000
Jeko-1	MCL	119	4,781

GI₅₀: the concentration that causes 50% growth inhibition.