Evaluation and binding site determination of microtubule PET tracers [¹¹C]MPC-6827 and [¹¹C]HD-800 in cancer cell lines

Objectives: Loss of microtubule (MT), a cytoskeleton polymer is implicated in the pathogenesis of cancers, and microtubule-targeted agents (MTA) are potential chemotherapeutic. However, most MTAs developed so far are substrates of P-glycoprotein (p-GP) and multidrug resistance protein 1 (MDR1), preventing their use in the treatment of brain malignancies or as in vivo PET imaging agents in the brain. [¹¹C]MPC-6827 and [¹¹C]HD-800 are the first brain-penetrating MTA based radiotracers, that show excellent uptake and specific binding in rodent brain.^1,2 The objective of this study is to evaluate the specific and competitive binding of [¹¹C]MPC-6827 and [¹¹C]HD-800 in cell lines of central and periphery malignancies.

Methods: [¹¹C]MPC-6827 and [¹¹C]HD-800 were synthesized by reacting corresponding desmethyl precursors with [¹¹C]CH₃I in a GE-FX2MeI/FX2M radiochemistry module.^1,2 In vitro binding of radiotracers was determined in breast cancer MDA-MB-231, glioblastoma (GBM) patient derived tumor (GBM-PDX), GBM U251 and prostate cancer 3 (PC3) cell lines at 37 ^oC in quadruplicate at 5, 15, 30, 60, and 90 minute incubation time. The nonspecific bindings were determined by incubation with microtubule targeting agents MPC-6827, HD-800, colchicine, paclitaxel and docetaxel (5.0 μM). The bindings are expressed as %ID/mg of protein present in each well, with p values ≤ 0.05 considered statistically significant.

Results: Among the tested cells, MDA-MB-231 cells show highest uptake and specific binding (~90%) for both tracers. [¹¹C]MPC-6827 binds to GBM- PDX, U251 and PC3 cells with ~50%, 40% and 35% specific binding respectively. Under identical conditions, [¹¹C]HD-800 shows 60% specific binding with GBM- PDX and PC3 cells. [¹¹C]MPC-6827 and [¹¹C]HD-800 displayed equivalent proportion (~70%) of binding to taxane and colchicine sites of MTs in MDA-MB-231 cells, whereas, docetaxel displaced >85% binding of both tracers.

Conclusions: We demonstrated specific bindings of [¹¹C]MPC-6827 and [¹¹C]HD-800 in breast cancer, GBM, and prostate cancer cell lines. Both tracers showed binding to taxane and colchicine sites of microtubule in MAD-MB-231 cells. This may be an advantage of these tracers for favoring high signal to noise ratio for in vivo imaging in cancer animal models. Combined with the previously established BBB permeability and in vivo binding specificity, [¹¹C]MPC-6827 and [¹¹C]HD-800 can be potential radiotracers for in vivo preclinical imaging of central and periphery cancers using PET. Research Support: Diane Goldberg Foundation (NYSPI/CUMC) and WFSM CTSA TIP (UL1TR001420) References: 1. Wang X-F. et.al. J. Med. Chem. 2014, 57, 1390-1402; Cui M-T. et.al., J. Med. Chem., 2017, 60, 5586. 2. Kumar J.S.D. et al., J. Med. Chem., 2018, 61, 2118-2120; Solingapuram Sai K. K. et.al., ACS Med Chem Lett., 2018, 78, 108-115.