1) gamma-butyrobetaine is not meldonium: if I read the structures correctly, meldonium has an NH group where the former has a CH2 group, adjacent to the terminal N(CH3)3 group. 2) Hydrated forms of drugs will act identically once they are absorbed into the bloodstream. However, the speed of solution, and hence rate of absorption, of anhydrous materials may be substantially greater than that of hydrates (the heat of hydration helps drive the dissolution). Additionally, anhydrous forms may be more stable in formulations. So no, you cannot deem them equivalent, without demonstrating this (in vivo AND in vitro, because the controlling agencies will want both kinds of data for an application, and the latter data for ongoing quality control batch testing).

Thank you very much for such a useful explanation

OK. you see, midronate (meldonium) and its hydrated form are characterized by respective CAS #-s. Additionally, both ones are categorized as equal pharmaceutical substances - there is a proof at the attached link. But unhydrated meldonium's highly hygroscopic. Thus, acc. to the GMP it can't be used as a substance to produce the finished dosage form (capsules). But this trouble can be avoided via addition of corresponding adjuvants. Furthermore, the primary package (Al-blister card) prevents absorption of water. The trouble is to argue the equality of these pharmaceutical compounds in terms of (non)-hydrated forms. I'll be glad to see opinions of experts having to do with pharmaceutics. Deep scientific disscussions are not required :-)

Anyway, all opinions are useful!

http://www.chemicalbook.com/Search_EN.aspx?keyword=Mildronate

Once the active entity is absorbed into the system, it's likely to be bioequivalent. BUT, various salts of it can have both different intrinsic (i.e. from a pure substance compressed disc face) speeds of dissolution and solubilities. Formation of a docusate (dioctylsulfosuccinate), for example, will greatly retard and impede the solution of a cationic drug; a maleate less so, and a hydrochloride not at all. The more hydrophobic the counterion (adjuvant, to you), the worse the dissolution, but the less hydroscopic the salt. You should know the equilibrium moisture uptake curves (moisture vs. %RH) for all your formulation ingredients; the least hydrophobic combination compatible with easy manufacture and stability of your dosage form will afford the quickest (in vitro) release profile, if that is what you want (this would help ensure bioequivalence of an oral tablet to an intragastric solution infusion, for example).