Multi-coordinate gold-phosphine compounds and method of use

A compound having the following formula or a pharmaceutically acceptable salt thereof:

• wherein R1 and R2, taken together with the gold (Au) to which they are bound, form a polycyclic moiety selected from the group consisting of:
• and
• X is CH2, O, NH, or C═O;
• Y is C or N;
• each R3 is independently selected from the group consisting of H, alkyl, and aryl;
• R4 is H, alkyl, aryl, or taken together with two of the atoms of the ring to which it is bound, forms a 6-membered ring;
• R5 is H, alkyl, or aryl;
• and
• R6 is H, CH3, COH, a targeting ligand, or an affinity tag.