Discovery of New Orally Active Phosphodiesterase (PDE4) Inhibitors

Abstract

Phosphodiesterase 4 (PDE4) is an enzyme that is responsible for the inactivation of cyclic adenosine monophosphate (cAMP).1,2) The anti-inflammatory effects of PDE4 inhibitors have been well established in a variety of animal models. 3)However, clinical use of these inhibitors has not been achieved yet because of dose-limiting side effects such as nausea and vomiting that restrict therapeutic application. 4)In previous reports, we have described the design and synthesis of bicyclo[3 · 3 · 0]octane derivatives 5) and piperidine derivatives, 6) which are interesting new classes of PDE4 inhibitors with therapeutic potential. Structure activity relationship (SAR) studies of PDE4 inhibitors based on the structural features of Ariflo 1 7) have revealed that its further modification yields a series of PDE4 inhibitors with an improved therapeutic index relative to the classical inhibitor, rolipram 2 (Fig. 1). To create efficient new PDE4 inhibitors with a higher therapeutic potential, discovery and evaluation of a new chemical lead with a completely different structure was considered to provide another approach to chemical modification. Here we report on the discovery of two new orally active pyrazolopyridines 5 and 9 (Fig. 2). SAR data are also presented. ChemistryThe synthesis of the test compounds listed in Tables 1-3 is outlined in Chart 1. [8][9][10][11] Compounds 27a, d-e, g-i, p, x, and z were commercially available. The other 1-methylpyrazoles, 27c and 27f, were prepared by condensation of methylhydrazine to the corresponding Michael acceptors 33 and 34, respectively. 3-Methylpyrazoles, 27j-o, q-w, and y were prepared from 35 and the corresponding hydrazine derivatives. Michael addition of 27a-z to diethyl ethoxymethylenemalonate gave 28a-z, respectively, after which ring closure was accomplished with phosphorus oxychloride under reflux to afford chlorides 29a-z, respectively. Alkaline hydrolysis of 29a-z gave carboxylic acids 30a-z, respectively. Heating of 30a-z with thionyl chloride, followed by treatment with aqueous ammonia, resulted in production of the amides 31a-z, respectively. Replacement of the chloro moiety of 31a-z with appropriate anilines led to the test compounds 5 and 8-26. N-Methylation of the 3-methoxyaniline moiety afforded 7. Compounds 3, 4, and 6 were prepared from 31a according to the procedure described for preparation of 5 from 31a. Results and DiscussionA series of pyrazolopyridine derivatives were synthesized and evaluated for inhibition of phosphodiesterase type 4 prepared from U937 cells, 12) which were derived from human monocytes. The results of the assays are expressed as IC 50 values, i.e., the test compound concentration that achieved 50% inhibition relative to the vehicle. Test compounds were also evaluated for inhibition of LPS-induced production of tumor necrosis factor-a (TNF-a) in rats.13) The results are expressed as ID 50 values, i.e., the dose that showed 50% inhibition relative to the vehicle.During the course of screening of PDE4 inhibitors, compound 3 showed moderate ...