AIDS-028537 AIDS028537 N-((1S)-1-Carbamoyl-2-methylpropyl)(4S,5S,2R)-5-[(tert-butoxy)carbonylamino]-4-hydroxy-6-phenyl-2-benzylhexanamide SB-109026 Val-Phe-Phe-HOCH2CH2 isostere analog