Abstract A226: ImmunoPET with ⁸⁹Zr‐DFO‐trastuzumab: Monitoring the pharmacodynamic effects on HER2/neu expression by Hsp90 inhibition in vivo

Objectives: The positron‐emitting radionuclide ⁸⁹Zr (t_1/2 = 3.17 days) has been isolated in high specific‐activity and was used to prepare ⁸⁹Zr‐radiolabeled monoclonal antibodies (mAbs) for use as immunoPET imaging agents. The work presented describes the preparation of ⁸⁹Zr‐DFO‐trastuzumab for use as a radiotracer for delineating HER2/neu positive tumors for the non‐invasive imaging of breast cancer in vivo. In addition, pharmacokinetic studies on HER2/neu expression levels in response to administration of therapeutic doses of PU‐H71 (a specific inhibitor of heat‐shock protein 90 [Hsp90]) were conducted.

Methods: ⁸⁹Zr was prepared via the ⁸⁹Y(p, n)⁸⁹Zr transmutation with high radiochemical yields (1.52±0.11 mCi/ Ah) and purity (>99.99%). Trastuzumab was functionalized with the tris‐hydroxamate chelate, desferrioxamine B (DFO) and radiolabeled with [⁸⁹Zr]Zr‐oxalate at room temperature. ImmunoPET imaging experiments in female, athymic nu/nu mice bearing sub‐cutaneous BT‐474 (HER2/neu positive) and/or MDA‐MB‐468 (HER2/neu negative) were conducted. The change in ⁸⁹Zr‐DFO‐trastuzumab tissue uptake in response to high‐ and low‐specific‐activity formulations and co‐administration of PU‐H71 (a Hsp90 chaperone inhibitor) was measured by using both acute biodistribution studies and Western blot analysis.

Results: ⁸⁹Zr‐DFO‐trastuzumab radiolabeling proceeded in high radiochemical yield and specific‐activity of 2.82±0.05 mCi/mg. In vitro assays demonstrated >99% radiochemical purity with an immunoreactive fraction of 0.87±7. In vivo biodistribution experiments revealed high and specific tumor 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 quantitative biodistribution and ⁸⁹Zr‐DFO‐trastuzumab immunoPET imaging. 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 studies (73.64 ± 12.17 %ID/g) and Western blot analysis demonstrated that HER2/neu expression recovered to baseline levels.

Conclusions: The results indicate that ⁸⁹Zr‐DFO‐trastuzumab provides quantitative and highly‐specific delineation of HER2/neu positive tumors and may be used to monitor treatment with Hsp90 inhibitors. Efforts towards the “current Good Manufacturing Process” (cGMP) production and clinical translation of ⁸⁹Zr‐DFO‐trastuzumab are underway.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A226.