Rosiglitazone maleate: combating insulin resistance.

The Pharmacophore

Rosiglitazone belongs to a class of oral anti-diabetic agents called the thiazolidinediones which seem to be ideally suited for the treatment of type 2 diabetes..  All agents of this class have a thiazolidine-2-4 dione structure as shown in fig 1.  The various agents of this class differ in their side chains which alter their pharmacologic and side-effect profiles.

                                 

   pharmacophore                                                                                                                                                                                                                                    

                                                                                                                                  fig. 1  the glitazones

Further modifications to rosiglitazone to create molecules such as

PMT-13 which are even more potent have been carried out; such

molecules still retain the thiazolidine-2-4 dione structure.

Non-TZD insulin sensitizers are now being synthesized based on an                                 PMT-13

alkoxy-propionic class of molecules, but these bind to the isoform PPARα rather than PPARγ.

The PPARγ Receptor:  Rosiglitazone’s means to the end

The peroxisome proliferator-activated receptors (PPARs) form a subfamily of the nuclear receptor superfamily.  The three isoforms of PPARs are ligand-dependent transcription factors that regulate target gene expression by binding to specific peroxisome proliferators response elements (PPREs) in enhancer sites of regulated genes.  Each receptor binds to its PPRE as a heterodimer with a retinoid X receptor (RXR).  Upon binding an agonist, the conformation of a PPAR is altered and stabilized such that a binding cleft is created and recruitment of transcriptional coactivators occurs.  The result is an increase in gene transcription.  Through experiments involving dominant negative mutations in human PPARγ it was established that the receptor is indeed implicated in the cause of insulin resistance.  Rosiglitazone and other thiazolidinediones are specific high-affinity ligands for PPARγ.

Binding of Rosiglitazone and PPARγ

Rosiglitazone binding with the PPAR-gamma LBD and SRC-1 in the ternary complex. a, Ribbons drawing showing the ternary complex of PPAR-gamma LBD, BRL 49653, and the LXXLL helix domain of SRC-1. Residues around K301 and E471 that form the 'charged clamp' are red, and the LXXLL SRC-1 helix is green. Rosiglitazone (stick diagram) binds in a deep cavity of the protein and provides a network of polar interactions that include the AF-2 domain. b, The secondary-structure elements are shown as a ribbon drawing, with amino acids involved in ligand binding labeled. 8

From a study conducted by Young et al. using radioiodinated ligand, it was determined that rosiglitazone bound to PPARγ effectively only in the S-conformation. The IC50 value of the S-entantiomer was 2.1 nm compared to 2770nm of the R-enantiomer. The acidic TZD heterocycle forms hydrogen bonds with His 323 on helix 5 and Tyr-473 on the AF2 helix.

Pharmacological mechanism of action

Rosiglitazone reduces insulin resistance by increasing insulin-dependent glucose disposal in skeletal muscle cells and reducing hepatic glucose output by the liver.  In subjects with dominant negative PPARγ mutations, adipocyte differentiation was inhibited indicating that PPARγ is necessary in the process of adipocyte differentiation.7 Also, the receptor is present in much greater quantities in adipose tissue than in skeletal or liver tissue.  The primary effect of PPARγ is on adipose tissue with secondary insulin-sensitizing effects on skeletal muscle and liver cells.  By stimulating glucose uptake into adipocytes through the glucose transporter GLUT-4, PPARγ causes liver and muscle cells to be more sensitive to existing levels of glucose i.e. it decreases insulin resistance.

Combination therapy:  Two soldiers are better than one.

        Rosiglitazone monotherapy is effective in patients with type 2 diabetes; in studies conducted, it reduced fasting plasma glucose levels by 3.22 mmol/L in 2 mg doses (bd) and by 4.22 mmol/L in 4 mg doses (bd).  β cell function was estimated to be improved over baseline by up to 60%.

        Although effective in monotherapy, the insulin sensitizer is often used in conjunction with sulphonylureas or metformin.  Sulphonylureas stimulate insulin secretion from β cells and thus treat the relative or absolute insulin deficiency of Type 2 diabetes rather than insulin resistance.  Studies demonstrated that endogenous fasting insulin concentrations with rosiglitazone (2mg bd) + sulphonylurea were 6.4 pmol/L lower than those in patients undergoing treatment with just sulphonylurea. Metformin, a biguanide, was used in a 12-week trial of rosiglitazone combination therapy; fasting glucose levels decreased from 213±46 to 152±35 mg/dL (p<0.005).  Because the mechanism of rosiglitazone differs from those of sulphonylurea and metformin, the effects of a combination of the two are additive, possibly synergistic.  In addition to drug combination therapy, type 2 diabetes is treated through lifestyle modifications such as weight loss and increased pharmacologic agents which decrease the body’s requirement for insulin.

Synergism

RXR ligands have been shown to be effective in activating PPARγ.  This is because PPARγ forms a heterodimer with the retinoid X receptor (RXR) that can be activated by both PPARγ and RXR-specific ligands.  Experiments were conducted to test whether LG100268, an RXR ligand, could enhance transcriptional activation by mutant PPARγ.  At moderate concentrations, a combination of PPARγ and RXR ligands induced significanlty greater transcriptional activation than either ligand alone indicating the possibility of synergistic effects.  Exercise stimulates glucose uptake by muscle cells with normal insulin sensitivity; rosiglitazone therapy in conjunction with exercise improves this synergic action.

Metabolism:  The fate of Rosiglitazone as it journeys through the human body.

Rosiglitazone is extensively metabolized; no unchanged drug was detected in the urine in studies conducted using 14C-labeled rosiglitazone.  It was rapidly cleared from the plasma in all subjects, being quantifiable only up to 24 hours after dosing.  N-demethylation and hydroxylation followed by conjugation with sulfate and glucoronic acid proved to be the major routes of metabolism.  In vitro data show that rosiglitazone is predominantly metabolized by the cytochrome P450(CYP)

isoenzyme 2C8 with CYP2C9 serving as a minor pathway.  The metabolites formed are active but have significantly less activity than the parent compound.  Below is a scheme proposed

for the metabolism of rosiglitazone in humans.  

M10 and M4 together accounted for approximately 35% of the dose excreted over 8 days.18  The scheme proposed is closely similar to that proposed for metabolism in rats and dogs.  According to Bolton et al., phase I metabolism in the rat and dog resulted in ring hydroxylation, N-demethylation and oxidative removal of the pyridinylamino function to yield a phenoxyacid derivative just as in the proposed scheme for metabolism in humans.  There were differences in species in the persistence of the circulating metabolites (measured as total radioactivity), but rosiglitazone’s principal metabolites were accurately predicted from preclinical studies.  Unlike the preclinical species, however, the phenoxyacetic acid metabolite M1 was a minor route of elimination in humans, accounting for less than 4% of the dose.18

Prodrug

A prodrug of rosiglitazone was not found.  This result seems reasonable as the pharmacokinetics of rosiglitazone are within an optimum range without modification.  It is already 99% bioavailable and none of its metabolites are toxic.  Search was conducted with keywords: avandia prodrug, rosiglitazone prodrug, diabetes prodrug, thiazolidinedione prodrug with the search engines PubMed, All Ovid, Google, Lexis-Nexis, ISI Web of Science, Medline, EMBASE Drugs and Pharmacology, and Academic Search Elite.

Possible Prodrug

Cytochrome P-450, an electron donor protein for several oxygenase enzymes found on the endoplasmic reticulum of most eukaryotic cells, can oxidize tertiary amines.  A carbinolamine is formed which readily decomposes to the secondary amine with loss of formaldehyde.  By this mechanism a possible prodrug would be rosiglitazone with the nitrogen

of the thiazole methylated.  The methylation prevents hydrogen

bonding making the molecule more lipophilic.  However, since the H-bonding is necessary for binding to PPARγ, P-450 must demethylate the prodrug before it can be effective.  This might delay the onset of action of rosiglitazone which would be useful in some circumstances.  The mechanism by which cytochrome P-450 would demethylate the possible prodrug is outlined below.

                                                                                                     

                                               

                                                     

       

Side Effects

In 26-week clinical trials, the mean weight gain in patients treated with rosiglitazone (8mg daily) monotherapy was 3.5kg. Edema was reported in 4.8% of patients receiving rosiglitazone vs 1.3% of patients on placebo. Decreases in hemoglobin and hematocrit of ≤ 1.0 g/dL and ≤3.3% respectively were observed in clinical trials of rosiglitazone monotherapy as well as in combination with other hypoglycemic agents.  There was also a slight decrease in white blood cell count which is probably related to the increased plasma volume.  In placebo-controlled trials, 0.2% of rosiglitazone-treated patients have reversible elevations in ALT (>3 times the upper limit of normal), compared with 0.5% of patients on active comparator agents.  Headaches, back pain and a slight cough are also minor side effects of rosiglitazone treatment.

 Tolerance to Rosiglitazone

No indications of tolerance to rosiglitazone due to effects of the drug itself were found.  In studies conducted regarding the effects of thiazolidinediones, 75% of the patients exhibited glucose-lowering effects while 25% did not.  Analysis of the individual data revealed that those that did not respond to the drug had the lowest levels of insulin secretion at the onset of the study.  This indicates that rosiglitazone is not effective in the absence of adequate levels of insulin.  Other factors that decrease glucose tolerance i.e. increase insulin resistance will cause rosiglitazone to be less effective.  In a study conducted by Gerben et al., the effects of caffeine on whole-body insulin sensitivity were observed.  The calculated insulin sensitivity during caffeine administration was .39 ±0.04 compared with 0.46 ± 0.04 μmol/kg in the placebo.  This decrease in insulin sensitivity of ~15% is close in magnitude to the increase in insulin sensitivity obtained by rosiglitazone and may thus be involved in seemed tolerance.

Searches were conducted on PubMed, All Ovid, Google, Lexis-Nexis, ISI Web of Science, Medline, EMBASE Drugs and Pharmacology, and Academic Search Elite with key words rosiglitazone tolerance, decreased effects of rosiglitazone (thiazolinediones), increasing insulin resistance, avandia tolerance.

Conclusion

Rosiglitazone, developed through a “screen to lead” technique, by SmithKlineBeecham Pharmaceuticals is now an effective drug against NIDDM.  It has been used to treat over 2,500,000 patients as of 2001.1

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