Bis(heteroaryl)piperazine (BHAP) reverse transcriptase inhibitors: Structure-activity relationships of novel substituted indole analogues and the identification of 1-[(5-methanesulfonamido-1H-indol-2-yl)-carbonyl]-4- [3-[(1-methylethyl)amino]-pyridinyl]piperazine monomethanesulfonate (U- 90152S), a second-generation clinical candidate