The invention provides a compound for use in medicine, the compound being a compound of the formula (VI0) or a salt, solvate, tautomer or N-oxide thereof: wherein the bicyclic group: is selected from the structures C1, C5 and C6: wherein n is 0, 1, 2 or 3; R1 is hydrogen, hydroxy, or O—Rz; R2a is hydroxy, methoxy or O—Rz; provided that at least one of R1 and R2a is O—Rz; Rz is Lp-Rp1; SO3H; a glucuronide residue; a mono-, di- or tripeptide residue; or Lp is a bond, C═O, (C═O)O, (C═O)NRp1 or S(O)xNRp1; x is 1 or 2; Rp1 is hydrogen or a an optionally substituted C1-25 hydrocarbyl group containing 0, 1 or 2 carbocyclic rings and 0, 1, 2, 3, 4, 5 or 6 carbon-carbon multiple bonds, provided that Rp1 is not hydrogen when Lp is a bond, C═O or (C═O)O; and provided also that O—Rz does not contain an O—O moiety; and excluding compounds wherein R1 is hydroxy and R2a is methoxy; Rp2 and Rp3 are the same or different and each is a group Rp1; and R3, R4a, R8 and R10 are defined in the claims.The compounds of formula (VI0) are pro-drugs of parent compounds wherein R1 and/or R2a are hydroxy, wherein the parent compounds have Hsp90 inhibiting activity.