Measuring the Pharmacodynamic Effects of a Novel Hsp90 Inhibitor on HER2/neu Expression in Mice Using 89Zr-DFO-Trastuzumab

Background: The positron-emitting radionuclide Zr-89 (t(1/2) = 3.17 days) was used to prepare Zr-89-radiolabeled trastuzumab for use as a radiotracer for characterizing HER2/neu-positive breast tumors. In addition, pharmacodynamic studies on HER2/neu expression levels in response to therapeutic doses of PU-H71 (a specific inhibitor of heat-shock protein 90 [Hsp90]) were conducted. Methodology/Principal Findings: Trastuzumab was functionalized with desferrioxamine B (DFO) and radiolabeled with [Zr-89] Zr-oxalate at room temperature using modified literature methods. ImmunoPET and biodistribution experiments in female, athymic nu/nu mice bearing sub-cutaneous BT-474 (HER2/neu positive) and/or MDA-MB-468 (HER2/neu negative) tumor xenografts were conducted. The change in Zr-89-DFO-trastuzumab tissue uptake in response to high-and low-specific-activity formulations and co-administration of PU-H71 was evaluated by biodistribution studies, Western blot analysis and immunoPET. Zr-89-DFO-trastuzumab radiolabeling proceeded in high radiochemical yield and specific-activity 104.3+/-2.1 MBq/mg (2.82+/-0.05 mCi/mg of mAb). In vitro assays demonstrated >99% radiochemical purity with an immunoreactive fraction of 0.87+/-0.07. In vivo biodistribution experiments revealed high specific BT-474 uptake after 24, 48 and 72 h (64.68+/-13.06% ID/g; 71.71+/-10.35% ID/g and 85.18+/-11.10% ID/g, respectively) with retention of activity for over 120 h. Pre-treatment with PU-H71 was followed by biodistribution studies and immunoPET of Zr-89-DFO-trastuzumab. Expression levels of HER2/neu were modulated during the first 24 and 48 h post-administration (29.75+/-4.43% ID/g and 41.42+/-3.64% ID/g, respectively). By 72 h radiotracer uptake (73.64+/-12.17% ID/g) and Western blot analysis demonstrated that HER2/neu expression recovered to baseline levels. Conclusions/Significance: The results indicate that Zr-89-DFO-trastuzumab provides quantitative and highly-specific delineation of HER2/neu positive tumors, and has potential to be used to measure the efficacy of long-term treatment with Hsp90 inhibitors, like PU-H71, which display extended pharmacodynamic profiles.