Patent ID: 8758723

Claim:
A compound of formula (I): wherein A 1 , A 2 , A 3 , and A 4 are each independently —(CH 2 ) x —, wherein x=2-4; and R 1 , R 2 , R 3 , R 4 are each independently H, alkyl, substituted alkyl, -alkyl-COOH, a protecting group, a targeting ligand that targets an amino acid transporter, or a linker-targeting ligand that targets an amino acid transporter, provided that R 1 , R 2 , R 3 , and R 4 are not all H, not all the same alkyl radical, or not all —CH 2 COOH; wherein if A 1 =—(CH 2 ) 2 — and A 3 =—(CH 2 ) 2 —, and either A 2 and A 4 =—(CH 2 ) 2 —, or A 2 and A 4 =—(CH 2 ) 3 —, then at least one of R 1 , R 2 , R 3 , and R 4 is a targeting ligand that targets an amino acid transporter further wherein the targeting ligand is an amino acid or an amino acid derivative.