Abraxane-induced bone marrow CD11b⁺ myeloid cell depletion in tumor-bearing mice is visualized by μPET-CT with ⁶⁴Cu-labeled anti-CD11b and prevented by anti-CSF-1

To investigate the utility of noninvasive µPET-CT with 64Cu-DOTA-anti-CD11b (⁶⁴Cu-αCD11b) in assessing bone marrow status after anticancer therapies, and the protective role of anti-CSF-1 (αCSF-1) against bone marrow suppression induced by Abraxane. Methods: MDA-MB-435 tumor-bearing mice were treated with Abraxane, αCSF-1, or αCSF-1 plus Abraxane. µPET-CT and biodistribution of 64Cu-αCD11b were performed after intravenous injection of the radiotracer. Cells from mouse bone marrow and MDA-MB-435 tumor were analyzed by flow cytometry. A humanized αCSF-1 was investigated for its role in protecting bone marrow cells, using a transgenic mouse model that expresses functional human CSF-1. Results: μPET-CT showed that 64Cu-αCD11b had high uptake in the bone marrow and spleen of both normal and tumor-bearing mice. Abraxane significantly reduced 64Cu-αCD11b uptake in the bone marrow and spleen of treated mice compared to untreated mice. Interestingly, ⁶⁴Cu-αCD11b μPET-CT revealed that αCSF-1 alleviated the depletion of bone marrow cells by Abraxane. These changes in the bone marrow population of CD11b⁺ myeloid cells were confirmed by flow cytometry. Moreover, αCSF-1 potently enhanced tolerance of bone marrow granulocytic myeloid cells to Abraxane, decreased cell migration, and suppressed recruitment of myeloid cells to the tumor microenvironment. The humanized αCSF-1 also alleviated the effects of Abraxane on bone marrow cells in transgenic mice expressing human CSF-1, suggesting clinical relevance of αCSF-1 in prevention of bone marrow suppression in addition to its role in reducing tumor-infiltrating myeloid cells. Conclusions: Abraxane-induced bone marrow CD11b⁺ myeloid cell depletion in tumor-bearing mice could be noninvasively assessed by μPET-CT with 64Cu-αCD11b and prevented by αCSF-1.