Evaluation of the Lysophosphatidic Acid Receptor Type 1 Radioligand 11C-BMT-136088 for Lung Imaging in Rhesus Monkeys.

The lysophosphatidic acid receptor type 1 (LPA1) is 1 of 6 known receptors of the extracellular signaling molecule lysophosphatidic acid. It mediates effects such as cell proliferation, migration, and differentiation. In the lung, LPA1 is involved in pathways leading, after lung tissue injury, to pulmonary fibrosis instead of normal healing, by mediating fibroblast recruitment and vascular leakage. Thus, a LPA1 PET radiotracer may be useful for studying lung fibrosis or for developing LPA1-targeting drugs. We developed and evaluated the radiotracer 11C-BMT-136088 (1-(4'-(3-methyl-4-(((1(R)-(3-11C-methylphenyl)ethoxy)carbonyl)amino)isoxazol-5-yl)-[1,1'-biphenyl]-4-yl)cyclopropane-1-carboxylic acid) in rhesus monkeys to image LPA1 in the lung in vivo with PET. Methods: The study consisted of 3 parts: test-retest scans; self-saturation to estimate the tracer's in vivo dissociation constant, nondisplaceable volume of distribution (VND), and nondisplaceable binding potential (BPND); and dosimetry. In the first 2 parts, the radiotracer was administered using a bolus-plus-infusion protocol, the arterial input function was measured, and the animals underwent 2 scans per day separated by about 4 h. Lung regions of interest were segmented, and the tissue density estimated, from CT images. A fixed blood volume correction was applied. The tracer volume of distribution (VT) was estimated using multilinear analysis 1 (MA1) or equilibrium analysis (EA). Results:11C-BMT-136088 baseline VT was 1.83 ± 0.16 (MA1, n = 5) or 2.1 ± 0.55 (EA, n = 7) mL of plasma per gram of tissue in the left and right lung regions of interest, with a test-retest variability of -6% (MA1, n = 1) or -1% ± 14% (EA, n = 2). For the self-saturation study, 11C-BMT-136088 VND and BPND were estimated to be 0.9 ± 0.08 mL of plasma per gram of tissue and 1.1 ± 0.14, respectively. The unlabeled drug dose and plasma concentration leading to a 50% reduction of 11C-BMT-136088 specific binding were 73 ± 30 nmol/kg and 28 ± 12 nM, respectively. The average plasma free fraction was 0.2%; thus, the tracer's in vivo dissociation constant was estimated to be 55 pM. For the dosimetry study, the highest organ dose was in the liver (43.1 ± 4.9 and 68.9 ± 9.4 µSv/MBq in reference human male and female phantoms, respectively), and the effective dose equivalent was 6.9 ± 0.6 and 8.7 ± 0.6 µSv/MBq, respectively. Conclusion: Specific binding of 11C-BMT-136088 can be reliably measured to quantify LPA1 in the lungs of rhesus monkeys in vivo.

Amelioration of dermal fibrosis by genetic deletion or pharmacologic antagonism of lysophosphatidic acid receptor 1 in a mouse model of scleroderma.

OBJECTIVE: Scleroderma (systemic sclerosis [SSc]), is characterized by progressive multiorgan fibrosis. We recently implicated lysophosphatidic acid (LPA) in the pathogenesis of pulmonary fibrosis. The purpose of the present study was to investigate the roles of LPA and two of its receptors, LPA1 and LPA2, in dermal fibrosis in a mouse model of SSc. METHODS: Wild type (WT), and LPA1-knockout (KO) and LPA2-KO mice were injected subcutaneously with bleomycin or phosphate buffered saline (PBS) once daily for 28 days. Dermal thickness, collagen content, and numbers of cells positive for α-smooth muscle actin (α-SMA) or phospho-Smad2 were determined in bleomycin-injected and PBS-injected skin. In separate experiments, a novel selective LPA1 antagonist AM095 or vehicle alone was administered by oral gavage to C57BL/6 mice that were challenged with 28 daily injections of bleomycin or PBS. AM095 or vehicle treatments were initiated concurrently with, or 7 or 14 days after, the initiation of bleomycin and PBS injections and continued to the end of the experiments. Dermal thickness and collagen content were determined in injected skin. RESULTS: The LPA1 -KO mice were markedly resistant to bleomycin-induced increases in dermal thickness and collagen content, whereas the LPA2-KO mice were as susceptible as the WT mice. Bleomycin-induced increases in dermal α-SMA+ and phospho-Smad2+ cells were abrogated in LPA1-KO mice. Pharmacologic antagonism of LPA1 with AM095 significantly attenuated bleomycin-induced dermal fibrosis when administered according to either a preventive regimen or two therapeutic regimens. CONCLUSION: These results suggest that LPA/LPA1 pathway inhibition has the potential to be an effective new therapeutic strategy for SSc, and that LPA1 is an attractive pharmacologic target in dermal fibrosis.