Discovery and development of dipeptidyl peptidase-4 inhibitors
Inhibition of the DPP-4 enzyme prolongs and enhances the activity of incretins that play an important role in insulin secretion and blood glucose control regulation.
[1] Type 2 diabetes mellitus is a chronic metabolic disease that results from inability of the β-cells in the pancreas to secrete sufficient amounts of insulin to meet the body's needs.
Current treatments, other than insulin supplementation, are sometimes not sufficient to achieve control and may cause undesirable side effects, such as weight gain and hypoglycemia.
In recent years, new drugs have been developed, based on continuing research into the mechanism of insulin production and regulation of the metabolism of sugar in the body.
[5] In 1994, researchers from Zeria Pharmaceuticals unveiled cyanopyrrolidines with a nitrile function group that was assumed to form an imidate with the catalytic serine.
Concurrently other DPP-4 inhibitors without a nitrile group were published but they contained other serine-interacting motifs, e.g. boronic acids, phosphonates or diacyl hydroxylamines.
In 1995, Edwin B. Villhauer at Novartis started to explore N-substituted glycinyl-cyanopyrrolidines based on the fact that DPP-4 identifies N-methylglycine as an N-terminal amino acid.
[3][6] Fig.1: During a meal, the incretins glucagon-like peptide 1 (GLP-1) and glucose-dependent gastric inhibitory polypeptide (GIP) are released by the small intestine into the blood stream.
Inhibition of DPP-4 slows their inactivation, thereby potentiating their action, leading to lower plasma glucose levels, hence its utility in the treatment of type 2 diabetes.
Tissues which strongly express DPP-4 include the exocrine pancreas, sweat glands, salivary and mammary glands, thymus, lymph nodes, biliary tract, kidney, liver, placenta, uterus, prostate, skin, and the capillary bed of the gut mucosa (where most GLP-1 is inactivated locally).
Thus, preventing the degradation of the incretin hormones GLP-1 and GIP by inhibition of DPP-4 has potential as a therapeutic strategy in the treatment of type 2 diabetes.
A deep lipophilic pocket combined with several exposed aromatic side chains for achieving high affinity small molecule binding.
A significant solvent access that makes it possible to tune the physico-chemical properties of the inhibitors that leads to better pharmacokinetic behavior.
Existing X-ray structures show that there is not much difference in size and shape of the pocket that indicates that the S1-pocket has high specificity for proline residues[9][8] DPP-4 inhibitors usually have an electrophilic group that can interact with the hydroxyl of the catalytic serine in the active binding site (Figure 3).
The nitrile group forms reversible covalent bonds with the catalytically active serine hydroxyl (Ser630), i.e. cyanopyrrolidines are competitive inhibitors with slow dissociation kinetics.
Strict steric constraint exists around the pyrrolidine ring of cyanopyrrolidine-based inhibitors, with only hydrogen, fluoro, acetylene, nitrile, or methano substitution permitted.
In fact, substitution with quite big branched side chains, e.g. tert-butylglycin, normally increased activity and chemical stability, which could lead to longer-lasting inhibition of the DPP-4 enzyme.
That long-lasting action was most likely due to slow dissociation of the enzyme-inhibitor complex and an active oxide metabolite that undergoes enterohepatic circulation.
This prevention was successful when incorporating an amide group into a ring, creating a compound that kept the DPP-4 inhibitory activity that, did not undergo the intramolecular cyclization and was even more selective over different DPP enzymes.
The product, vildagliptin, was even more stable, undergoing intramolecular cyclization 30-times slower, and having high DPP-4 inhibitory activity and longer-lasting pharmacodynamic effect.
Because of that increased stability, the researchers continued their investigation on cis-4,5-methano cyanopyrrolidines and came across with a new adamantyl derivative, which showed extraordinary ex vivo DPP-4 inhibition in rat plasma.
Biological evaluations have shown that the S-configuration of the amino acid portion is essential for the inhibitory activity since the R-configuration showed reluctantly inhibition.
The most potent ketoazetidines and cyanoazetidines have large hydrophobic amino acid groups bound to the azetidine nitrogen and are active below 100nM.
When they started internal screening and medicinal chemistry program, two DPP-4 inhibitors were already in clinical trials, isoleucyl thiazolidide (P32/38) and NVP-DPP728 from Novartis.
When trying to stabilize the piperazine moiety, a group of bicyclic derivatives were made, which led to the identification of a potent and selective triazolopiperazine series.
Since sitagliptin has shown excellent selectivity and in vivo efficacy it urged researchers to inspect the new structure of DPP-4 inhibitors with appended β-amino acid moiety.
It was found that when replacing the quinazolinone with a pyrimidinedione, the metabolic stability was increased and the result was a potent, selective, bioavailable DPP-4 inhibitor named alogliptin.
Alogliptin has shown excellent inhibition of DPP-4 and extraordinary selectivity, greater than 10.000 fold over the closely related serine proteases DPP-8 and DPP-9.
[23] In January 2007 alogliptin was undergoing the phase III clinical trial and in October 2008 it was being reviewed by the U.S. Food and Drug Administration.
The quinazoline group undergoes a π-stacking interaction with the Trp629 residue The pharmacokinetic properties of sitagliptin and vildagliptin appear unaffected by age, sex or BMI.
