The invention relates to stable and substantially purified synthetic pharmaceutically acceptable acid addition salts of pyridazine compounds of the formula I wherein R10 is hydrogen, hydroxyl, alkyl, alkoxy, alkenyl, alkynyl, alkylene, aryl, heteroaryl, sulfonyl, sulfinyl, sulfenyl, amino, imino, azido, thiol, thioalkyl, thioalkoxy, thioaryl, nitro, cyano, halo, =0, ═S, carboxyl, carbonyl, carbamoyl, carboxamide, or phosphonate, and R11 is alkyl, alkoxy, alkenyl, alkynyl, alkylene, alkenylene, alkenyloxy, aryl, aryloxy, arylalkoxy, aroyl, heteroaryl, heterocyclic, acyl, acyloxy, sulfonyl, sulfinyl, sulfenyl, amino, imino, azido, thiol, thioalkyl, thioalkoxy, thioaryl, nitro, cyano, halo, =0, ═S, phosphonate, carboxyl, carbonyl, carbamoyl, carboxamide, or ureido. The invention also relates to formulations, dosage forms and compositions comprising the salts, and methods of using the salts, formulations, dosage forms and compositions.