Palovarotene appeared to cause small improvements in lung density and lung function relative to placebo, which although it failed to reach statistical significance, may indicate biological activity. 321 Other synthetic retinoids have been developed that specifically modulate the activity of RARs, CRABP1 and CRABP2, lecithin:retinol acetyltransferase, and CYP26 enzymes. 322 , 323 , 324 Proof-of-concept studies in lung cells in vitro could probe the efficacy of
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