Iodothyronamines are oxidatively deaminated to iodothyroacetic acids in vivo.

3-Iodothyronamine (T(1)AM) and 3,3',5-triiodothyroacetic acid (Triac) are bioactive metabolites of the hormone thyroxine (T(4)). In the present study, the ability of T(1)AM and 3,3',5-triiodothyronamine (T(3)AM) to be metabolized to 3-iodothyroacetic acid (TA(1)) and Triac, respectively, was investigated. Both T(1)AM and T(3)AM were converted to their respective iodinated thyroacetic acid analogues in both cell and tissue extracts. This conversion could be significantly inhibited with the monamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) inhibitor iproniazid. TA(1) was found to be present in trace quantities in human serum and in substantial levels in serum from T(1)AM-treated rats. These results demonstrate that iodothyronamines are substrates for amine oxidases and that this metabolism may be the source of the corresponding endogenous arylacetic acid products Triac and TA(1).

Substrate activity screening (SAS): a general procedure for the preparation and screening of a fragment-based non-peptidic protease substrate library for inhibitor discovery.

Substrate activity screening (SAS) is a fragment-based method for the rapid development of novel substrates and their conversion into non-peptidic inhibitors of Cys and Ser proteases. The method consists of three steps: (i) a library of N-acyl aminocoumarins with diverse, low-molecular-weight N-acyl groups is screened to identify protease substrates using a simple fluorescence-based assay; (ii) the identified N-acyl aminocoumarin substrates are optimized by rapid analog synthesis and evaluation; and (iii) the optimized substrates are converted into inhibitors by direct replacement of the aminocoumarin with known mechanism-based pharmacophores. This protocol describes a general procedure for the solid-phase synthesis of a library of N-acyl aminocoumarin substrates and the screening procedure to identify weak binding substrates.

Identification of selective, nonpeptidic nitrile inhibitors of cathepsin s using the substrate activity screening method.

The substrate activity screening method, a substrate-based fragment identification and optimization method for the development of enzyme inhibitors, was previously applied to cathepsin S to obtain low nanomolar 1,4-disubstituted-1,2,3-triazole-based aldehyde inhibitors (Wood, W. J. L.; Patterson, A. W.; Tsuruoka, H.; Jain, R. K.; Ellman, J. A. J. Am. Chem. Soc. 2005, 127, 15521-15527). Replacement of the metabolically labile aldehyde pharmacophore with the nitrile pharmacophore provided inhibitors with moderate potency for cathepsin S. The inhibitors showed good selectivity over cathepsins B and L but no selectivity over cathepsin K. X-ray structures of two crystal forms (1.5 and 1.9 A) of a complex between cathepsin S and a triazole inhibitor incorporating a chloromethyl ketone pharmacophore guided the design of triazole substrates with increased cleavage efficiency and selectivity for cathepsin S over cathepsins B, L, and K. Conversion of select substrates to nitrile inhibitors yielded a low molecular weight (414 Da) and potent (15 nM) cathepsin S inhibitor that showed >1000-fold selectivity over cathepsins B, L, and K.

Substrate activity screening: a fragment-based method for the rapid identification of nonpeptidic protease inhibitors.

A new fragment-based method for the rapid development of novel and distinct classes of nonpeptidic protease inhibitors, Substrate Activity Screening (SAS), is described. This method consists of three steps: (1) a library of N-acyl aminocoumarins with diverse, low molecular weight N-acyl groups is screened to identify protease substrates using a simple fluorescence-based assay, (2) the identified N-acyl aminocoumarin substrates are optimized by rapid analogue synthesis and evaluation, and (3) the optimized substrates are converted to inhibitors by direct replacement of the aminocoumarin with known mechanism-based pharmacophores. The SAS method was successfully applied to the cysteine protease cathepsin S, which is implicated in autoimmune diseases. Multiple distinct classes of nonpeptidic substrates were identified upon screening an N-acyl aminocoumarin library. Two of the nonpeptidic substrate classes were optimized to substrates with >8000-fold improvements in cleavage efficiency for each class. Select nonpeptidic substrates were then directly converted to low molecular weight, novel aldehyde inhibitors with nanomolar affinity to cathepsin S. This study demonstrates the unique characteristics and merits of this first substrate-based method for the rapid identification and optimization of weak fragments and provides the framework for the development of completely nonpeptidic inhibitors to many different proteases.

Synthesis of a diverse library of mechanism-based cysteine protease inhibitors.

We report improvements of our method for the solid-phase synthesis of mechanism-based mercaptomethyl ketone inhibitors of cysteine proteases (Lee, A.; Huang, L.; Ellman, J. A. J. Am. Chem. Soc. 1999, 121, 9907-9914). Specifically, Fmoc-protected chloromethyl ketones were used, rather than the Alloc-protected counterparts. In addition, we further demonstrated that diverse polar functionality can be incorporated at the R1', R1, and R2 sites, in contrast to our previous efforts, where primarily hydrophobic groups were incorporated at these positions. On the basis of these results, a 2016-membered library of potential mercaptomethyl ketone inhibitors was prepared that incorporated diverse functionality. The library was screened against cathepsin B, which is implicated in cancer, resulting in the identification of single-digit nanomolar inhibitors. Because of the diverse functionality incorporated in this library, it should be a rich source of potent inhibitors against many other cysteine proteases.