Complex sentence: As mentioned, the CCR5 receptor is a G-protein coupled receptor (GPCR) and before the discovery of CCR5's role in HIV infection many pharmaceutical companies had already built a substantial collection of compounds that target GPCRs.
Split into simpler sentences: As mentioned, the CCR5 receptor is a G-protein coupled receptor (GPCR). Before the discovery of CCR5's role in HIV infection, many pharmaceutical companies had already built a substantial collection of compounds that target GPCRs.