Quantitative Structure-Activity Relationships for γδ T Cell Activation by Bisphosphonates

γδ T cells are the first line of defense against many infectious organisms and are also involved in tumor cell surveillance and killing. They are stimulated by a broad range of small, phosphorus-containing antigens (phosphoantigens) as well as by the bisphosphonates commonly used in bone resorption therapy, such as pamidronate and risedronate. Here, we report the activation of γδ T cells by a broad range of bisphosphonates and develop a pharmacophore model for γδ T cell activation, in addition to using a comparative molecular similarity index analysis (CoMSIA) approach to make quantitative relationships between γδ T cell activation by bisphosphonates and their three-dimensional structures. The CoMSIA analyses yielded R² values of ∼0.8-0.9 and q² values of ∼0.5-0.6 for a training set of 45 compounds. Using an external test set, the activities (IC₅₀ values) of 16 compounds were predicted within a factor of 4.5, on average. The CoMSIA fields consisted of ∼40% hydrophobic, ∼40% electrostatic, and ∼20% steric interactions. Since bisphosphonates are known to be potent, nanomolar inhibitors of the mevalonate/isoprene pathway enzyme farnesyl pyrophosphate synthase (FPPS), we also compared the pharmacophores for γδ T cell activation with those for FPPS inhibition, using the Catalyst program. The pharmacophores for γδ T cell activation and FPPS inhibition both consisted of two negative ionizable groups, a positive charge feature and an endocyclic carbon feature, all having very similar spatial dispositions. In addition, the CoMSIA fields were quite similar to those found for FPPS inhibition by bisphosphonates. The activities of the bisphosphonates in γδ T cell activation were highly correlated with their activities in FPPS inhibition: R = 0.88, p = 0.002, versus a human recombinant FPPS (N = 9 compounds); R = 0.82, p < 0.0001, for an expressed Leishmania major FPPS (N = 45 compounds). The bisphosphonate γδ T cell activation pharmacophore differs considerably, however, from that reported previously for γδ T cell activation by phosphoantigens (Gossman, W.; Oldfield, E. J. Med. Chem. 2002, 45, 4868-4874), suggesting different primary targets for the two classes of compounds. The ability to quite accurately predict the activity of bisphosphonates as γδ T cell activators by using 3D QSAR techniques can be expected to help facilitate the design of additional bisphosphonates for potential use in immunotherapy.