[¹⁸F]ML-10 PET imaging fails to assess early response to neoadjuvant chemotherapy in a preclinical model of triple negative breast cancer

Pathological complete response to the neoadjuvant therapy (NAT) for triple negative breast cancer (TNBC) is predictive of prolonged patient survival. Methods for early evaluation of NAT efficiency are still needed, in order to rapidly adjust the therapeutic strategy in case of initial non-response. One option for this is molecular imaging of apoptosis induced by chemotherapy. Therefore, we investigated the capacity of [¹⁸F]ML-10 PET imaging, an apoptosis radiotracer, to detect tumor cell apoptosis and early predict the therapeutic response of human TNBC.

Initially, the induction of apoptosis by different therapies was quantified. We confirmed, in vitro, that paclitaxel or epirubicin, the fundamental cytotoxic drugs for breast cancer, induce apoptosis in TNBC cell lines. Exposure of TNBC models MDA-MB-231 and MDA-MB-468 to these drugs induced a significant increase (p < 0.01) of the apoptotic hallmarks: DNA fragmentation, membrane phospholipid scrambling, and PARP activation. Secondarily, apoptotic fraction was compared to the intracellular accumulation of the radiotracer. [¹⁸F]ML-10 accumulated in the apoptotic cells after 72 h of treatment by paclitaxel in vitro; this accumulation positively correlated with the apoptotic fraction. In vivo, [¹⁸F]ML-10 was rapidly cleared from the nontarget organs and mainly eliminated by the kidneys. Comparison of the in vivo [¹⁸F]FDG, [¹⁸F]FMISO, and [¹⁸F]ML-10 uptakes revealed that the tumor accumulation of [¹⁸F]ML-10 was directly related to the tumor hypoxia level. Finally, after the in vivo treatment of TNBC murine xenografts by paclitaxel, apoptosis was well induced, as demonstrated by the cleaved caspase-3 levels; however, no significant increase of [¹⁸F]ML-10 accumulation in the tumors was observed, either on day 3 or day 6 after the end of the treatment.

These results highlighted that PET imaging using [¹⁸F]ML-10 allows the visualization of apoptotic cells in TNBC models. Nevertheless, the increase of the chemotherapy-induced apoptotic response when using paclitaxel could not be assessed using this radiotracer in our mouse model.