CLAIMS

1. A pharmaceutical composition for protecting cells from the cytotoxicity of β-amyloid protein, which comprises a compound of the formula:

wherein R represents a lower alkyl;

R represents a hydrogen, an optionally substituted alkyl or an optionally substituted alkenyl;

R and R each represents an optionally substituted lower alkyl or a lower alkoxy, or R and R form, taken together, an optionally substituted butadienylene; and

X 1 and X 2 each represents an optionally esterified or etherified hydroxy, or a salt thereof.

2. A composition according to claim 1, which is for the prophylaxis or treatment of Parkinson's disease, Huntington's chorea or Creutzfeldt-Jacob disease.

3. A composition according to claim 1, wherein R is a Cj_₄ alkyl;

R is (a) hydrogen, (b) a Cι_₂₂ alkyl which may be substituted by 1 to 10 substituents selected from the group consisting of (i) C ₄ alkyl, (ii) hydroxy, (iii) oxo, (iv) amino, (v) mono-Cj.j alkylamino, (vi) di-Cι_₆ alkylamino, (vii) carboxy, (viii) C-.i, alkoxy-carbonyl, (ix) C₆.₁₄ aryl which may be substituted by 1 or 2 substituents selected from the group consisting of C,.,, alkyl, hydroxy, carboxy and C^ alkoxy-carbonyl, (x) 5- or 6-membered heterocyclic group which may be substituted by 1 or 2 substituents selected from the group consisting of C^ alkyl, hydroxy, carboxy and C,_₆ alkoxy-carbonyl, and (xi) halogen, or (c) a C .₁₅

alkenyl which may be substituted by 1 to 10 substituents selected from the group consisting of (i) Cj.-, alkyl, (ii) hydroxy, (iii) oxo, (iv) amino, (v) mono-C^ alkylamino, (vi) di-Cj.s alkylamino, (vii) carboxy, (viii) Cj._A alkoxy-carbonyl, (ix) C₆._u aryl which may be substituted by 1 or 2 substituents selected from the group consisting of C^ alkyl, hydroxy, carboxy and C^ alkoxy-carbonyl, (x) 5- or 6- membered heterocyclic group which may be substituted by 1 or 2 substituents selected from the group consisting of Cj.₄ alkyl, hydroxy, carboxy and C ₆ alkoxy-carbonyl, and (xi) halogen;

R³ and R⁴ each is a C^ alkyl which may be substituted by 1 to 3 substituents selected from the group consisting of hydroxy, halogen, nitro, C^ alkyl which may be halogenated, carboxy, C^ alkoxy-carbonyl, 3- pyridyl, 1-imidazolyl and 5-thiazolyl or a C^ alkoxy; or R³ and R⁴ form, taken together with the respective adjacent carbon atoms, a benzene ring which may be substituted by 1 to 3 substituents selected from the group consisting of C^ alkyl, C^ alkoxy, hydroxy, nitro and halogen; and

X¹ and X² each is hydroxy, C₂.₁₀ alkanoyl, benzoyl, nicotinoyl which may be quanternized, succinic acid he i-acyl, C^ alkoxy, C₇_₁₃ aralkyloxy, tetrahydropyranyloxy or tetrahydrofuryloxy.

4. A composition according to claim 1, wherein R is a -__.3 alkyl, R² is a C₆.₁₄ alkyl substituted by hydroxy, R³ and R each is a Cι_₃ alkoxy, and X¹ and X each is hydroxy.

5. A composition according to claim 1, which comprises a compound of the formula:

wherein all symbols are same as claim 1 or a salt.

6. A composition according to claim 1, which comprises idebenone.

7. A method of protecting cells from the cytotoxicity of β-amyloid protein in a mammal, which comprises administering to such mammal an effective amount of a compound of the formula:

wherein R represents a lower alkyl;

R represents a hydrogen, an optionally substituted alkyl or an optionally substituted alkenyl;

R³ and R⁴ each represents an optionally substituted lower alkyl or a lower alkoxy, or R and R* form, taken together, a butadienylene; and

X and X each represents an optionally esterified or etherified hydroxy, or a salt thereof with a pharmaceutically acceptable excipient, carrier or diluent.

8. Use of a compound of the formula:

wherein R represents a lower alkyl;

R² represents a hydrogen, an optionally substituted alkyl or an optionally substituted alkenyl;

R³ and R⁴ each represents an optionally substituted

₃ 4 lower alkyl or a lower alkoxy, or R and R form, taken together, a butadienylene; and

X¹ and X² each represents an optionally esterified or etherified hydroxy, or a salt thereof for the manufacture of a composition for protecting cells from the cytotoxicity of β-amyloid protein.

9. Use according to claim 8, which is for the prophylaxis or treatment of Parkinson's disease,

Huntington's chorea or Creutzfeldt-Jacob disease.