The disclosure relates to (8S,9R)-5-fluoro-8-(4-fluorophenyl)-9-(1-methyl-1H-1,2,4-triazol-5-yl)-8,9-dihydro-2H-pyrido[4,3,2-de]phthalazin-3(7H)-one tosylate salt (abstract figure), specifically in a crystalline form. The crystalline form has a solid state 13C NMR spectrum with peaks at 143.2, 136.0, 131.8, 123.9, 112.2, 105.2 and 100.3 ppm ± 0.2 ppm； a differential scanning calorimetry thermogram having an endotherm with a maximum at between about 320°C and about 335 °C； a thermogravimetric analysis thermogram indicative of an unsolvated material； a dynamic vapour sorption isotherm plot which does not exhibit a significant weight change from 0 to 95% relative humidity； an X-ray powder diffraction pattern comprising characteristic peaks expressed in d-values (Å) of about 11.9, 5.9, 4.9, 4.4, 4.3, 3.9, and 3.7； and an X-ray powder diffraction pattern comprising peaks at 2 &thetas； angle degrees ± 0.2 2&thetas； angle degrees of 7.4, 15.1, 18.1, 20.1, 20.4, 22.6 and 24.0. The disclosure also relates to methods of synthesising and crystallising the compound and methods of treating as disease or condition, including cancer, using the compound.
