In Vitro and In Vivo Characterization of 89Zirconium-Labeled Lintuzumab Molecule

Objective: Positron emission tomography (PET) imaging is a powerful non-invasive method to determine the in vivo behavior of biomolecules. Determining biodistribution and pharmacokinetic (PK) properties of targeted therapeutics can enable a better understanding of in vivo drug mechanisms such as tumor uptake, off target accumulation and clearance. Zirconium-89 (Zr-89) is a readily available tetravalent PET-enabling radiometal that has been used to evaluate the biodistribution and PK of monoclonal antibodies. In the current study, we performed in vitro and in vivo characterization of Zr-89-lintuzumab, a radiolabeled anti-CD33 antibody, as a model to evaluate the in vivo binding properties in preclinical models of AML. Methods: Lintuzumab was conjugated to p-SCN-Bn-deferoxamine (DFO) and labeled with Zr-89 using a 5:1 mu Ci:mu g specific activity at 37 degrees C for 1h. The biological activity of Zr-89-lintuzumab was evaluated in a panel of CD33 positive cells using flow cytometry. Fox Chase SCID mice were injected with 2 x 10(6) OCI-AML3 cells into the right flank. After 12 days, a cohort of mice (n = 4) were injected with Zr-89-lintuzumab via tail vein. PET/CT scans of mice were acquired on days 1, 2, 3 and 7 post Zr-89-lintuzumab injection. To demonstrate Zr-89-lintuzumab specific binding to CD33 expressing tumors in vivo, a blocking study was performed. This cohort of mice (n = 4) was injected with native lintuzumab and 24 h later Zr-89-lintuzumab was administered. This group was imaged 3 and 7 days after injection of Zr-89-lintuzumab. A full ex vivo biodistribution study on both cohorts was performed on day 7. The results from the PET image and ex vivo biodistribution studies were compared. Results: Lintuzumab was successfully radiolabeled with Zr-89 resulting in a 99% radiochemical yield. The Zr-89-lintuzumab radioconjugate specifically binds CD33 positive cells in a similar manner to native lintuzumab as observed by flow cytometry. PET imaging revealed high accumulation of Zr-89-lintuzumab in OCI-AML3 tumors within 24h post-injection of the radioconjugate. The Zr-89-lintuzumab high tumor uptake remains for up to 7 days. Tumor analysis of the PET data using volume of interest (VOI) showed significant blocking of Zr-89-lintuzumab in the group pre-treated with native lintuzumab (pre-blocked group), thus indicating specific targeting of CD33 on OCI-AML3 cells in vivo. The tumor uptake findings from the PET imaging study are in agreement with those from the ex vivo biodistribution results. Conclusions: PET imaging of Zr-89-lintuzumab shows high specific uptake in CD33 positive human OCI-AML3 tumors. The results from the image study agree with the observations from the ex vivo biodistribution study. Our findings collectively suggest that PET imaging using Zr-89-lintuzumab could be a powerful drug development tool to evaluate binding properties of anti-CD33 monoclonal antibodies in preclinical cancer models.

Automated summary

PET imaging with Zr-89-lintuzumab demonstrates high specific uptake in CD33 positive human OCI-AML3 tumors. The findings from the image study are consistent with the observations from the ex vivo biodistribution study. Our results collectively suggest that PET imaging using Zr-89-lintuzumab could be a powerful drug development tool to evaluate binding properties of anti-CD33 monoclonal antibodies in preclinical cancer models.