GW-501516 is a PPAR modulator. There are other PPAR modulators on the market right now including Pioglitazone and Rioglitazone. Side effects of these drugs include cardiovascular events (heart attack) and increased risk of cancer. I believe the government has even pulled some of these drugs off the market in the United States due to their adverse effect risks.
The chemical structures and mechanism of action of these compounds are very similar to GW 501516. It may also follow that the side effects may be similar.
Use of experimental compounds is at your own risk.