A group of novel compounds which can pass through the blood-brain barrier (BBB) with a drug carried thereon and reside in the brain to release the drug and a group of known compounds having the above characteristics. More specifically, a compound represented by general formula (Ia) and a salt thereof, wherein R¹ represents hydroxy, carboxy, amino which may be substituted by C₁-C₆ alkyl, or C₁-C₆ alkyl which may be substituted by a 5-to 7-membered saturated heterocyclic group, R² represents hydrogen or C₁-C₆ alkyl, R³ represents hydrogen or C₁-C₆ alkyl which may be hydroxylated, R⁴ represents hydrogen or C₁-C₆ alkyl, R⁵ represents amino acid residue, or -S-R⁶ or -CO-R⁶ (wherein R⁶ represents C₁-C₁₄ alkyl which may be substituted by a 5- to 7-membered saturated heterocyclic group, C₁-C₁₄ alkyl, C₂-C₆ alkenyl, aryl or a 5- to 7-membered saturated ring group), or a group represented by general formula (IVa), wherein R¹, R², R³ and R⁴ are each as defined above, and symbol ......... represents either a single or a double bond, provided that at least one of R¹, R³ and R⁵ represents hydroxy, carboxy or amino.