SPC3 stands for Synthetic Polymeric Construction No. 3. It is a synthetic peptide that mimics the tip of the V3 loop, part of HIVs gp120 envelope protein.

In the test tube it appears to prevent HIV from infecting lymphocytes and macrophages and to prevent the formation of syncytia. It is not immunogenic, meaning that the immune system does not recognise it as `foreign and try to remove it from the body. It acts by binding with the glycolipid fusion receptors on human cells, not the CD4 molecule.

A randomised phase III study among sex workers in Thailand found that a microbicide based on SPC3, a product called Advantage S, did not prevent HIV infection.