indole 2 carboxylic acid preferred examples of the biohydrolyzable esters include c 1 c 6 alkyl esters such as methyl ester ethyl ester etc c 3 c 9 acyloxyalkyl ester such as acetoxymethyl ester 1 acetoxyethyl ester pivaloyloxymethyl ester etc c 3 c 9 alkoxycarbonyloxyallkyl esters such as isopropoxycarbonyloxymethyl ester etc 2 oxo 1 3 dioxol 4 yl c 1 c 6 alkyl esters such as 2 oxo 1 3 dioxol 4 yl methyl ester etc morpholinoethyl ester and the like the pharmaceutically acceptable salts of the tricyclic indole 2 carboxylic acid of the formula 1 or the prodrug thereof include for example salts with an inorganic acid such as hydrochloride hydrobromide nitrate sulfate phosphate and the like salts with an organic acid such as formate acetate trifluoroacetate propionate lactate tartrate oxalate fumarate maleate citrate malonate mathanesulfonate and the like salts with inorganic metal such as alkaline metal salts lithium salt sodium salt potassium salt etc alkaline earth metal salts magnesium salt calcium salt barium salt etc an aluminum salt and the like salts with an organic base such as basic amino acid salts arginine salt lysine salt etc triethylammonium salt tetrabutylammonium salt pyridinium salt pyrrolidinium salt piperidinium salt and the like or an ammonium salt preferred examples are salts with an inorganic acid and salts with an organic acid preferably hydrochloride the solvates of the tricyclic indole 2 carboxylic acid of the formula 1 or of the prodrug or pharmaceutically acceptable salt thereof include for example hydrates alcoholates such as ethanolate and the like a preferred example is the hydrate.