Porphyrin compounds and compositions useful for treating cancer

A compound of Formula III

• or a pharmaceutically acceptable salt thereof, wherein
• an A1, A2, A3 and A4 are each covalently attached to a porphyrin ring of Formula III and A1, A2, A3, and A4 are independently selected from a substituted aromatic ring or a six membered heteroaromatic ring containing a single nitrogen atom at the 2, 3 or 4 position relative to the porphyrin ring;
• B1 is a covalent linker moiety which connects A1 to a cytotoxic agent Z1 and is selected from
• wherein n is selected from 1-12;
• and
• the Z1 is a cytotoxic agent selected from