Acid addition salts of 5-aminolevulinic acid or its derivatives

An acid addition salt of a 5-aminolevulinic acid (5-ALA) ester with an acid which has a pKa of about 5 or less, wherein either said acid is nitric acid, and said 5-ALA ester is a compound of formula X:

• R22N—CH2COCH2—CH2CO—OR1 (X)
• wherein R1 represents a straight-chained C1-6 alkyl group substituted by an aryl group, and
• R2 each independently represents a hydrogen atom or an optionally substituted straight-chained, branched or cyclic alkyl group which may optionally be interrupted by one or more —O—, —NR3—, —S— or —PR3— groups;
• and
• R3 is a hydrogen atom or a C1-6 alkyl group,
• or said acid is a sulfonic acid or a sulfonic acid derivative and said 5-ALA ester is a compound of formula XI:
• R52N—CH2COCH2—CH2CO—OR4 (XI)
• wherein R4 represents a straight-chained C1-6 alkyl group optionally substituted by an aryl group, and
• R5 each independently represents a hydrogen atom or an optionally substituted straight-chained, branched or cyclic alkyl group which may optionally be interrupted by one or more —O—, —NR6—, —S— or —PR6— groups;
• and
• R6 is a hydrogen atom or a C1-6 alkyl group.