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Only partial deficiency/inhibition of P-glycoprotein (P-gp, ABCB1) function at the blood-brain barrier (BBB) is likely to occur in pathophysiological situations or drug-drug interactions. This raises questions regarding the sensitivity of available PET imaging probes to detect moderate changes in P-gp function at the living BBB. In vitro, the half-maximum inhibitory concentration (IC 50 ) of the potent P-gp inhibitor tariquidar in P-gp-overexpressing cells was significantly different using either [ 11 C]verapamil (44 nM), [ 11 C] N-desmethyl-loperamide (19 nM) or [ 11 C]metoclopramide (4 nM) as substrate probes. In vivo PET imaging in rats showed that the half-maximum inhibition of P-gp-mediated efflux of [ 11 C]metoclopramide, achieved using 1 mg/kg tariquidar ( in vivo IC 50  = 82 nM in plasma), increased brain exposure by 2.1-fold for [ 11 C]metoclopramide (p   0.05, n = 4). This comparative study points to differences in the “vulnerability” to P-gp inhibition among radiolabeled substrates, which were apparently unrelated to their “avidity” (maximal response to P-gp inhibition). Herein, we advocate that partial inhibition of transporter function, in addition to complete inhibition, should be a primary criterion of evaluation regarding the sensitivity of radiolabeled substrates to detect moderate but physiologically-relevant changes in transporter function in vivo.

Comparative vulnerability of PET radioligands to partial inhibition of P-glycoprotein at the blood-brain barrier: A criterion of choice?