Abstract 2344: Modeling 1st- and 2nd-line therapy of BRAF mutant melanoma using a novel BRAF inhibitor with DFG-out binding mode.
Authors: Waizenegger Irene C. , Baum Anke , Stadtmüller Heinz , Steurer Steffen , Bader Gerd , Garin-Chesa Pilar , Schweifer Norbert , Bernthaler Andreas , Haslinger Christian , Schaaf Otmar , Mousa Sien , Colbatzky Florian , Adolf Günther R. 
Published: August 2014

The BRAF inhibitor vemurafenib (VEM) was approved for treatment of patients with BRAFV600E melanoma based on an ORR of 48% and a PFS of 5.3 months. Phase III data reported for the BRAF inhibitor dabrafenib (DAB) as well as the MEK inhibitor trametinib (TRA) in BRAFmut melanoma indicate similar outcomes. Phase I/II data published recently for a combination of DAB and TRA suggest improved efficacy compared with single-agent therapy; nevertheless, the short duration of responses even for the combination points to an urgent need for further improvement.

BI 882370, a potent and selective B/CRAF inhibitor, binds to the DFG-out (inactive) conformation of BRAF, whereas VEM as well as DAB occupy the DFG-in (active) conformation. BI 882370 inhibited proliferation of BRAFmut melanoma cell lines with about 100x higher potency (EC50 1 - 10 nM) than VEM; BRAFwt cell lines were not affected at 1 μM.

For studies in nude mouse xenograft models, VEM and DAB were dosed to provide exposures achieved in patients; TRA was used at the MTD for mice, resulting in exposures at or above the exposures tolerated by humans. In the G361 melanoma model (BRAFV600E), 1st line treatment with 25 mg/kg BI 882370 administered twice daily (bid) resulted in partial regression of all tumors, superior to results achieved by VEM, DAB as well as their combination.

To model clinical resistance, a large cohort of A375 melanoma (BRAFV600E) bearing mice was treated with VEM; without exception, all tumors responded with partial regression and their median volume reached a nadir on day 11. Thereafter, the majority of tumors resumed growth in spite of continued treatment, whereas tumors in a subset of animals regressed further. Progressing tumors were randomized to 2nd line therapy on day 36 (median volume 280 mm3). TRA did not show any efficacy in this resistant population. BI 882370 induced tumor regression, however, resistance developed within 3 weeks. BI 882370 in combination with TRA resulted in more pronounced regressions and resistance was not observed until the end of the experiment, 6 weeks after initiation of 2nd line therapy.

Importantly, in multiple in vivo studies performed to date, mice treated with BI 882370 at doses of 25 mg/kg bid for several weeks did not show any body weight loss or clinical signs of intolerability. Mice treated for 2 weeks were euthanized and major organs subjected to histological analysis. No changes were observed in any of the animals; in particular, there was no evidence for epidermal hyperplasia. Further, a pilot study in male rats dosed at 25 mg/kg bid for 2 weeks did not result in any toxicologically relevant findings in terms of clinical chemistry, hematology, pathology and toxicogenomics of liver and skin.

Our results suggest that compared to 1st generation BRAF inhibitors, BI 882370 may provide an improved therapeutic window, enabling more pronounced and longer-lasting pathway suppression and thus resulting in improved efficacy.

Citation Format: Irene C. Waizenegger, Anke Baum, Heinz Stadtmüller, Steffen Steurer, Gerd Bader, Pilar Garin-Chesa, Norbert Schweifer, Andreas Bernthaler, Christian Haslinger, Otmar Schaaf, Sien Mousa, Florian Colbatzky, Günther R. Adolf. Modeling 1st- and 2nd-line therapy of BRAF mutant melanoma using a novel BRAF inhibitor with DFG-out binding mode. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2344. doi:10.1158/1538-7445.AM2013-2344