A new ester of a known medicine with a carboxylic acid chain is patentable if

The Explanation to section 3(d) treats salts, esters and other derivatives of a known substance as "the same substance, unless they differ significantly in properties with regard to efficacy". For a medicine this means therapeutic efficacy, as the Supreme Court held in Novartis AG v. Union of India (2013); better solubility or a higher melting point are physical properties that do not by themselves meet section 3(d).