DETECTION OF IN-VIVO P-GLYCOPROTEIN INHIBITION BY PSC-833 USING TC-99M SESTAMIBI

Tc-99m sestamibi is a substrate of P-glycoprotein (Pgp) that has been proposed for use as a functional imaging agent for the multidrug resis tance-1 phenotype, In vitro, retention of sestamibi by cells that over express Pgp can be modified by the presence of Pgp antagonists, In a P hase I trial of the Pgp reversal agent PSC 833, we show that the effec ts of this reversal agent can also be demonstrated in patients, Nine p atients with metastatic renal carcinoma were studied three times: at b aseline, approximately 1 day after vinblastine infusion, and while on PSC 833, One patient with metastatic adrenocortical cancer was also st udied, Time activity curves and areas under the curve (AUCs) were obta ined for tumor, liver, lung, and myocardium, and organ:heart AUC ratio s were generated, With PSC 833, tumor visualization was enhanced, and statistically significant increases were found in AUC ratios for tumor and liver compared to baseline, For the liver, significant difference s were also found between the vinblastine versus PSC 833 scans but not between the baseline versus vinblastine scans, This study demonstrate s that sestamibi retention by tumor and liver is altered in the presen ce of the reversal agent PSC 833, presumably reflecting inhibition of Pgp, Thus, sestamibi may be useful in vivo as a means of monitoring th e effects of this and other reversal agents on various tumors and norm al tissues that express Pgp.