compounds having the general formula : ## str1 ## wherein each of r 1 and r 2 independently represents hydrogen or alkyl of 1 to 4 carbon atoms ; a represents -- o -- or -- cr 4 r 5 , where r 4 and r 5 are defined as for r 1 or r 2 ; r 3 represents an adamantyl group ; and py represents a 3 - or 4 - pyridyl group , as free bases or their pharmaceutically acceptable salts are useful in treating androgen - dependent , especially prostatic , cancer .