Salts of an FGFR inhibitor

A salt which is an acid salt of 3-(2,6-difluoro-3,5-dimethoxyphenyl) -1-ethyl-8-(morpholin-4-ylmethyl)-1,3,4,7-tetrahydro-2H-pyrrolo[3′, 2′:5,6]pyrido[4,3-d]pyrimidin-2-one, having the structure:

• or a hydrate or solvate thereof, wherein the acid is selected from L-(+)-tartaric acid and D-(−)-tartaric acid, wherein the salt is crystalline, and wherein:
• the salt is 3-(2,6-difluoro-3,5-dimethoxyphenyl)-1-ethyl-8-(morpholin-4-ylmethyl)-1,3,4, 7-tetrahydro-2H-pyrrolo[3′,2′:5,6]pyrido[4,3-d]pyrimidin-2-one L-(+)-tartrate having at least one characteristic X-ray powder diffraction peak selected from about 3.8, about 8.5, about 9.1, about 10.3, and about 12.0 degrees 2-theta;
• or
• the salt is 3-(2,6-difluoro-3,5-dimethoxyphenyl)-1-ethyl-8-(morpholin-4-ylmethyl)-1,3,4, 7-tetrahydro-2H-pyrrolo[3′,2′:5,6]pyrido[4,3-d]pyrimidin-2-one D-(−)-tartrate having at least one characteristic XRPD peaks selected from about 7.4, about 10.5, about 12.9, about 13.6, about 14.8, and about 16.4 degrees 2-theta.