Determination of plasma concentrations of losartan in patients by HPLC using solid phase extraction and UV detection

PURPOSE

To establish a HPLC assay for plasma losartan and its active metabolite EXP3174 to facilitate clinical pharmacokinetic studies.

METHODS

the HPLC system consisted of a 250 x 2 mm i.d. C18 reversed phase column preceded by a 4 x 4 mm guard column, a UV detector set at 254 nm, and an integrator. The mobile phase was a mixture of 0.01 M ammonium phosphate: acetonitrile: methanol (6:3:1) containing 0.02 % sodium azide and 0.04% TEA, with pH adjusted to 3.2. The system was operated isocratically at ambient temperature at a flow rate of 0.3 ml/min. Losartan and its active metabolite EXP3174 were extracted from plasma using C2 bonded silica gel standard solid phase extraction.

RESULTS

recoveries of losartan and EXP3174 from plasma were greater than 70%. Using 0.5 ml of plasma sample, standard curves were linear from 10 to 300 ng/ml (r2 = 0.996 and 0.997 for losartan and EXP 3174, respectively). Sensitivity of the assay was < 10 ng/ml. Intra-and inter-assay variations were < 10 and 15%. respectively. The assay has been successfully applied to measuring plasma concentrations of losartan and EXP3174 in patients receiving a daily dose of losartan (50-100 mg).

CONCLUSION

The HPLC assay has adequate sensitivity, reproducibility, and specificity for clinical pharmacokinetic studies.

Newly emerging pharmacologic differences in angiotensin II receptor blockers

Several angiotensin II receptor blockers (ARB) are currently available for the treatment of hypertension. These drugs share a common mechanism of action-antagonism of angiotensin II AT1 receptors; however, their receptor binding kinetics differ. Candesartan has a higher affinity for the AT1 receptor than all the other ARB. In addition, candesartan and irbesartan block the AT1 receptor with insurmountable antagonism, whereas losartan, valsartan, and eprosartan are competitive antagonists. The pharmacokinetics of these ARB also differ in terms of oral bioavailability, rate of absorption, metabolism, and route and rate of elimination. Both losartan potassium and candesartan cilexetil are prodrugs; however, losartan is partially converted into EXP3174 in the liver, whereas candesartan cilexetil is converted completely into candesartan during gastrointestinal absorption. On the basis of elimination half-lives, losartan, valsartan, and eprosartan may be classified as shorter acting and candesartan cilexetil and irbesartan as longer acting. Each drug effectively lowers blood pressure during once daily administration to patients with mild to moderate hypertension, with candesartan cilexetil requiring the lowest dosage and providing dose-dependent efficacy. Initial comparative clinical trials suggest that both candesartan cilexetil and irbesartan in the doses used are significantly more effective than losartan in lowering trough sitting diastolic blood pressure. It remains to be determined, however, whether the observed pharmacologic and pharmacokinetic differences among the members of the ARB class will have a clinically significant impact on long-term cardiovascular outcomes and reductions of cardiovascular mortality.

Interactions of angiotensin II non-peptide AT(1) antagonist losartan with phospholipid membranes studied by combined use of differential scanning calorimetry and electron spin resonance spectroscopy

We used differential scanning calorimetry (DSC) and electron spin resonance (ESR) spectroscopy to investigate the interactions of Losartan, a potent, orally active Angiotensin II AT(1) receptor antagonist with phospholipid membranes. DSC results showed that Losartan sensitively affected the chain-melting behavior of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) bilayer membranes. ESR spectroscopy showed that phosphatidylcholines spin-labeled at the 5-position of the sn-2 acyl chain (n-PCSL with n=5), incorporated either in DMPC or DPPC bilayers containing Losartan, were restricted in motion both in the gel and in the liquid-crystalline membrane phases, indicating a location of the antagonist close to the interfacial region of the phosphatidylcholine bilayer. At high drug concentrations (mole fraction >/= x=0.60), the decrease in chain mobility registered by 5-PCSL in fluid-phase membranes is smaller than that found at lower concentrations, whereas that registered by 14-PCSL is further increased. This indicates a different mode of interaction with Losartan at high concentrations, possibly arising from a location deeper within the bilayer. Additionally, Losartan reduced the spin-spin broadening of 12-PCSL spin labels in the gel-phase of DMPC and DPPC bilayers. As a conclusion, our study has shown that Losartan interacts with phospholipid membranes by affecting both their thermotropic behavior and molecular mobility.

Applications of modulated differential scanning calorimetry in preformulation studies

Characterization of the thermal properties of active pharmaceutical ingredients is critical in the selection of appropriate physical forms for development and defining proper manufacturing, handling and storage conditions of those chemical entities. Modulated differential scanning calorimetry (MDSC) has proven to be an effective tool in the thorough characterization of thermal behavior of compounds in preformulation studies. Selected applications of MDSC for various preclinical compounds are presented, thereby demonstrating the utility of this analytical method in the determination of glass transitions, characterization of desolvation and degradation processes as well as in the study of polymorphic transformations and crystallizations.

An effort to understand the molecular basis of hypertension through the study of conformational analysis of losartan and sarmesin using a combination of nuclear magnetic resonance spectroscopy and theoretical calculations

Losartan is the first recently approved drug against hypertension disease that competes with the biological action of angiotensin II (AII) at the AT1 receptor. Its design was based on the mimicry of the C-terminal segment of AII. Due to the biological significance of Losartan, its structure elucidation and conformational properties are reported as determined by NMR spectroscopy and computational analysis. In addition, molecular modeling of the peptide Sarmesin [Sar1Tyr(OMe)4AII], a competitive antagonist of AII, was also developed based on NMR and computational analysis data. Sarmesin's C-terminal was used as a template for superimposition with specific molecular features of interest in the structure of Losartan such as the conformation of biphenyltetrazole, the n-butyl chain, and the orientation of hydroxymethylimidazole relative to the biphenyl template. The major conclusions derived from this study are the following: (a) Sarmesin, like the AII superagonist [Sar1]AII, adopts a conformation which keeps in close proximity the key amino acids Sar1 (or Arg2)-Tyr(OMe)4-His6-Phe8. (b) Losartan favors a low-energy conformation in which imidazole and tetrazole rings are placed in the opposite site relative to the spacer phenyl ring plane; the hydroxymethyl group is placed away from the spacer phenyl ring, the alkyl chain is oriented above the spacer phenyl ring, and the two phenyl rings deviate approximately 60 degrees from being coplanar. (c) Overlay of the C-terminal region of Sarmesin with Losartan using equivalent groups revealed an excellent match. (d) Interestingly, the matching between enantiomeric structures of Losartan was not equivalent, proposing that the chirality of this molecule is significant in order to exert its biological activity. These findings open a new avenue for synthetic chemists to design and synthesize peptidomimetic drugs based on the C-terminal segment of the proposed model of Sarmesin. The new candidate drug molecules are not restricted to structurally resemble Losartan as the design is hitherto focused.

Design and synthesis of nonpeptide angiotensin II receptor antagonists featuring acyclic imidazole-mimicking structural units

Extensive molecular modelling studies, including conformational analysis and the comparison of molecular electrostatic potential distributions, wee used to evaluate structural parameters of new antagonists containing acyclic replacements of the N = C-N imidazole region. The synthesis and the biological screening of a series of acyl biphenyltetrazole derivatives were planned and realized to gain an insight into the structure-activity relationships of this unusual class of Angiotensin II antagonists.

Determination of losartan and its degradates in COZAAR tablets by reversed-phase high-performance thin-layer chromatography

Losartan potassium is an angiotensin II receptor blocker. It has been formulated and marketed as a tablet dosage from (COZAAR). A reversed-phase high-performance thin-layer chromatography method has been validated and shown to be sensitive, efficient, and reliable, and can be used as an excellent alternative to the HPLC stability testing of losartan potassium in COZAAR tablets.

Phosphodiesterase inhibitory properties of losartan. Design and synthesis of new lead compounds

A 4-centre PDE4 pharmacophore search has been carried out in several 3D-databases containing compounds belonging to different therapeutic areas. Losartan, an angiotensin-II antagonist, has been identified as a new lead compound for developing PDE4 inhibitors. New families of compounds derived from losartan has been synthesized and their PDE inhibition has been measured.

Superposition of three-dimensional chemical structures allowing for conformational flexibility by a hybrid method

The superposition of three-dimensional structures is the first task in the evaluation of the largest common three-dimensional substructure of a set of molecules. This is an important step in the identification of a pharmacophoric pattern for molecules that bind to the same receptor. The superposition method described here combines a genetic algorithm with a numerical optimization method. A major goal is to adequately address the conformational flexibility of ligand molecules. The genetic algorithm optimizes in a nondeterministic process the size and the geometric fit of the substructures. The geometric fit is further improved by changing torsional angles combining the genetic algorithm and the directed tweak method. This directed tweak method is based on a numerical quasi-Newton optimization method. Only one starting conformation per molecule is necessary. Molecules having several rotatable bonds and quite different initial conformations are modified to find large structural similarities. A set of angiotensin II antagonists is investigated to illustrate the performance of the method.