Simultaneous determination of phentermine and topiramate in human plasma by liquid chromatography-tandem mass spectrometry with positive/negative ion-switching electrospray ionization and its application in pharmacokinetic study.

A new method for simultaneous determination of phentermine and topiramate by liquid chromatography/electrospray tandem mass spectrometry (LC/MS/MS) operated in positive and negative ionization switching modes was developed and validated. Protein precipitation with acetonitrile was selected for sample preparation. Analyses were performed on a liquid chromatography system employing a Kromasil 60-5CN column (2.1 mm × 100 mm, 5 µm) and an isocratic elution with mixed solution of acetonitrile-20mM ammonium formate containing 0.3% formic acid (40:60, v/v), at a flow rate of 0.35 mL/min. Doxazosin mesylate and pioglitazone were used as the internal standard (IS) respectively for quantification. The determination was carried out on an API 4000 triple-quadrupole mass spectrometer operated in multiple reaction monitoring (MRM) mode using the following transitions monitored simultaneously: positive m/z 150.0/91.0 for phentermine, m/z 452.1/344.3 for doxazosin, and negative m/z 338.3/77.9 for topiramate, m/z 355.0/41.9 for pioglitazone. The method was validated to be linear over the concentration range of 1-800 ng mL(-1) for phentermine, 1-1000 ng mL(-1) for topiramate. Within- and between-day accuracy and precision of the validated method at three different concentration levels were within the acceptable limits of <15% at all concentrations. Blood samples were collected into heparinized tubes before and after administration. The simple and robust LC/MS/MS method was successfully applied for the simultaneous determination of phentermine and topiramate in a pharmacokinetic study in healthy male Chinese volunteers.

Investigating interactions between phentermine, dexfenfluramine, and 5-HT2C agonists, on food intake in the rat.

RATIONALE: Synergistic or supra-additive interactions between the anorectics (dex)fenfluramine and phentermine have been reported previously in the rat and in the clinic. Studies with 5-HT2C antagonists and 5-HT2C knockouts have demonstrated dexfenfluramine hypophagia in the rodent to be mediated by actions at the 5-HT2C receptor. Given the recent FDA approval of the selective 5-HT2C agonist lorcaserin (BELVIQ®) for weight management, we investigated the interaction between phentermine and 5-HT2C agonists on food intake. OBJECTIVES: This study aims to confirm dexfenfluramine-phentermine (dex-phen) synergy in a rat food intake assay, to extend these findings to other 5-HT2C agonists, and to determine whether pharmacokinetic interactions could explain synergistic findings with particular drug combinations. METHODS: Isobolographic analyses were performed in which phentermine was paired with either dexfenfluramine, the 5-HT2C agonist AR630, or the 5-HT2C agonist lorcaserin, and inhibition of food intake measured in the rat. Subsequent studies assessed these same phentermine-drug pair combinations spanning both the full effect range and a range of fixed ratio drug combinations. Satellite groups received single doses of each drug either alone or in combination with phentermine, and free brain concentrations were measured. RESULTS: Dex-phen synergy was confirmed in the rat and extended to the 5-HT2C agonist AR630. In contrast, although some synergistic interactions between lorcaserin and phentermine were observed, these combinations were largely additive. Synergistic interactions between phentermine and dexfenfluramine or AR630 were accompanied by combination-induced increases in brain levels of phentermine. CONCLUSIONS: Dex-phen synergy in the rat is caused by a pharmacokinetic interaction, resulting in increased central concentrations of phentermine.

Antiobesity pharmacotherapy: new drugs and emerging targets.

Obesity is a growing pandemic, and related health and economic costs are staggering. Pharmacotherapy, partnered with lifestyle modifications, forms the core of current strategies to reduce the burden of this disease and its sequelae. However, therapies targeting weight loss have a significant history of safety risks, including cardiovascular and psychiatric events. Here, evolving strategies for developing antiobesity therapies, including targets, mechanisms, and developmental status, are highlighted. Progress in this field is underscored by Belviq (lorcaserin) and Qsymia (phentermine/topiramate), the first agents in more than 10 years to achieve regulatory approval for chronic weight management in obese patients. On the horizon, novel insights into metabolism and energy homeostasis reveal guanosine 3',5'-cyclic monophosphate (cGMP) signaling circuits as emerging targets for antiobesity pharmacotherapy. These innovations in molecular discovery may elegantly align with practical off-the-shelf approaches, leveraging existing approved drugs that modulate cGMP levels for the management of obesity.

[Phentermine--a "weighty" or a dangerous substance?]. / Phentermin--ein "gewichtiger" oder gefährlicher Arzneistoff?

According to international surveys, the appetite suppressant phentermine has frequently been seized although its approval has been withdrawn in Germany. Phentermine is an isomer of methamphetamine though is not optically active such as e. g. amphetamine. The drug acts as a potent substrate at the norepinephrine transporter simultaneously promoting its release; it has a weaker activity at the dopamine transporter whereas its activity towards the serotonin transporter is negligible. Overall, its pharmacological action is comparable to that of amphetamine albeit less strong. Due to its declining effect with time and its addiction potential it has been recommended that phentermine should be used for a few weeks only. Phentermine hydrochloride is a readily soluble salt; absorption of the resinate compound is considerably slower. The drug is not extensively biotransformed; p- and N-hydroxyphentermine are the primary metabolites also being excreted as glucuronide conjugates. Gas chromatographic techniques to identify and to quantify phentermine in biological specimens are applicable following derivatization; however, liquid chromatography coupled to mass spectrometry is currently preferred for analysis of urine, serum or hair. Short-term clinical studies having been performed in the 80s and 90s revealed no serious harmful side effects. However, there are case reports proposing that phentermine usage might be associated with severe health risks due to hypertension, vasoconstriction and vasculopathy; in some individuals, mental illness had been observed. Apart from the legal consequences following purchase of drugs that have been withdrawn its user will simultaneously run serious and unpredictable health risks.

Phentermine/topiramate for the treatment of obesity.

OBJECTIVE: To review the pharmacology, efficacy, and safety of phentermine/topiramate (PHEN/TPM) in the management of obese patients. DATA SOURCES: MEDLINE (1966-July 2012) was searched using the terms weight loss, obesity, phentermine and topiramate, phentermine, topiramate, Qnexa, Qsymia, and VI-0521. Additionally, the new drug application and prescribing information for PHEN/TPM were retrieved. STUDY SELECTION/DATA EXTRACTION: All studies considering the pharmacology, efficacy, and safety of PHEN/TPM were reviewed with a focus on efficacy and safety data from Phase 3 trials. DATA SYNTHESIS: In 3 Phase 3 trials (EQUIP, CONQUER, and SEQUEL), treatment with PHEN/TPM consistently demonstrated statistically significant weight loss compared with placebo. After 56 weeks of treatment, percent weight loss achieved with PHEN/TPM was 10.6%, 8.4%, and 5.1% with 15/92 mg, 7.5/46 mg, and 3.75/23 mg, respectively (p < 0.0001). The 52-week extension study (SEQUEL) showed maintained weight loss over 2 years with 9.3% and 10.5% weight loss from baseline for 7.5/46 mg and 15/92 mg PHEN/TPM (p < 0.0001). A significantly higher proportion of patients achieved greater than 5%, 10%, or 15% weight loss with PHEN/TPM compared with placebo. Significant reductions in waist circumference, fasting triglycerides, and fasting glucoses were also attributable to PHEN/TPM. The drug was generally well tolerated in clinical trials. Adverse reactions occurring in 5% or more of study subjects included paresthesia, dizziness, dysgeusia, insomnia, constipation, and dry mouth. CONCLUSIONS: PHEN/TPM is a new, once-daily, controlled-release, combination weight-loss product approved as an adjunct to diet and exercise for chronic weight management of obese or overweight patients with weight-related comorbidities. PHEN/TPM is modestly effective and a viable option for patients interested in losing weight, although long-term safety data are lacking.

Pharmacokinetics, safety, and tolerability of phentermine in healthy participants receiving taranabant, a novel cannabinoid-1 receptor (CB1R) inverse agonist.

This study assessed the potential pharmacokinetic interaction and safety/tolerability of taranabant and phentermine coadministration. This was a randomized, double-blind, 3-panel, fixed-sequence study in healthy participants. Panels A, B, and C evaluated the safety/tolerability of phentermine 15 mg coadministered with taranabant 0.5, 1, and 2 mg for 7 days (panel A) and 28 days (panels B and C). In panels A and C, phentermine 15 mg was administered both with (7 days, panel A; 28 days, panel C) and without (7 days) taranabant 0.5 mg or 2 mg to evaluate pharmacokinetics. The primary endpoint was phentermine AUC(0-24 h) in panels A and C. Secondary endpoints were changes from baseline in blood pressure and heart rate for all panels. The geometric mean ratios and 90% confidence intervals for phentermine AUC(0-24 h) in the presence/absence of taranabant 0.5 mg and 2 mg were 1.08 (0.99, 1.17) and 1.04 (0.98, 1.10), respectively. No significant differences in blood pressure and heart rate were observed with any treatment versus placebo. Coadministration of taranabant 0.5 mg, 1 mg, and 2 mg with phentermine was well tolerated with no pharmacokinetic interaction and did not result in meaningful changes in blood pressure or heart rate versus placebo.

Liquid chromatography studies on the pharmacokinetics of phentermine and fenfluramine in brain and blood microdialysates after intraperitoneal administration to rats.

A highly sensitive and simple HPLC method with fluorescence detection for the determination of phentermine (Phen), fenfluramine (Fen) and norfenfluramine (Norf, the active metabolite of Fen) in rat brain and blood microdialysates has been developed. The brain and blood microdialysates were directly subjected to derivatization with 4-(4,5-diphenyl-1H-imidazol-2-yl) benzoyl chloride (DIB-Cl) in the presence of carbonate buffer (0.1 M, pH 9.0) at room temperature. The chromatographic conditions consisted of an ODS column and mobile phase composition of acetonitrile and water (65:35, v/v) with flow rate set at 1.0 ml/min. The detection was performed at excitation and emission wavelengths of 325 and 430 nm, respectively. Under these conditions, the DIB-derivatives of Phen, Fen and Norf were well separated and showed good linearities in the studied ranges (5-2000 nM for Phen and 10-2000 nM for Norf and Fen) with correlation coefficients greater than 0.999. The obtained detection limits were less than 23 fmol on column (for the three compounds) in both brain and blood microdialysates at a signal-to-noise ratio of 3 (S/N=3). The intra- and the inter-assay precisions were lower than 10%. The method coupled with microdialysis was applied for a pharmacokinetic drug-drug interaction study of Phen and Fen following individual and combined intraperitoneal administration to rats. In addition, since the role of protein binding in drug interactions can be quite involved, the method was applied for the determination of total and free Phen and Fen in rat plasma and ultrafiltrate, respectively. The results showed that Fen and/or Norf significantly altered the pharmacokinetic parameters of Phen in both blood and brain but did not alter its protein binding. On the other hand, there was no significant difference in the pharmacokinetics of Fen when administered with Phen.

Pharmacological management of obesity.

Obesity is a major chronic health problem in adults. It is a complex, multifactorial disorder characterised by excess accumulation of adipose tissue. It is associated with a number of complications including cardiovascular disease, hypertension, type 2 diabetes, dyslipidaemia and cancer. A weight loss in the order of 5-10% is associated with clinically meaningful reductions with respect to all comorbidities. Diet and exercise has been the cornerstone of weight management therapy, but this approach has limitations, especially for weight maintenance. Previous drugs used in obesity had serious side effects including valvular heart disease. However, recent drugs like orlistat and sibutramine have been rigorously tested and proven safe. Orlistat, a lipase inhibitor, inhibits absorption of dietary fat by approximately 30%. Taken with a hypocaloric diet, it produces and maintains clinically meaningful weight loss. Sibutramine is a centrally-acting agent which enhances satiety and thermogenesis by inhibiting serotonin and noradrenaline re-uptake. It is appropriate for patients who are unable to lose weight by lifestyle modification.

Chronic treatment with phentermine combined with fenfluramine lowers plasma serotonin.

As expected on the basis of published research in both humans and animals, treatment with phentermine/fenfluramine lowers plasma 5-hydroxytryptamine [corrected], whereas treatment with phentermine had no significant effect. In light of these findings, future research should focus on mechanisms other than increased plasma 5-hydroxytryptamine [corrected] to explain how fenfluramine increases the risk of primary pulmonary hypertension and valvular heart disease.

Steady-state pharmacokinetics of phentermine extended-release capsules.

Twenty-one healthy, caucasian, male volunteers completed this randomized single blind, multiple-dose, crossover bioavailability study during which either phentermine HCl capsules (Minobese Forte, reference product) or phentermine base capsules (Duromine, test product) were ingested once daily for 14 days. A washout period of 14 days was allowed between the two treatment phases. On profile days (day 14 of each treatment phase) subjects remained recumbent for 24 hours after drug administration. Serial venous blood samples were drawn over the 24 hour dosing interval for plasma phentermine assay by gas chromatography. The 90% confidence intervals for the "test/reference" mean ratios of the pharmacokinetic variables Cmax,norm, Cmin,norm, AUCnorm (normalized for difference in the dose of phentermine base), %PTF and T75% Cmax, all fell within the bioequivalence range of 80% to 125%. With the aid of trough plasma phentermine concentrations, it was established that steady-state was reached after 14 days of once daily administration of either product. Adverse events experienced on both treatments included prolonged or recurrent episodes of insomnia, nausea, headache, dry mouth and dizziness. No clinically relevant changes in clinical chemistry or hematology variables occurred during the study.

Structure-localization relationships of 11C-labeled phentermine derivatives: effect of aromatic substitution.

A series of phentermine analogs, including the unsubstituted, the para-F, -Cl, -Br and -I, and the meta-CF3 derivatives, were labeled by [11C]methylation and evaluated in rats to determine the structure-localization relationships for this class of regional cerebral blood flow imaging agents. All the phentermines were well-localized in the brain; however, only the para-substituted agents were well-retained. Localization in the nontarget tissue was affected by the lipophilicity of the substituent. Comparison with the radioiodinated analogs showed virtually identical results, which suggests that the compounds were not significantly metabolized. The agent with the best biodistribution characteristics was the N-[11C]methyl-p-iodophentermine, with the p-bromo analog almost equivalent.

The formation of metabolites of mephentermine by microsomal and cytosolic preparations of male Wistar rat livers.

1. Metabolites of mephentermine (MP), phentermine (Ph), p-hydroxy-MP, p-hydroxy-Ph, N-hydroxy-MP and N-hydroxy-Ph on incubation with rat liver microsomal and cytosolic preparations were identified by g.l.c. and g.l.c.-mass spectrometry. 2. Identification of the metabolites indicated the following new metabolic routes of MP: NADPH-dependent microsomal formation of p-hydroxy-MP from MP, of p-hydroxy-Ph from p-hydroxy-MP, and the NADH-dependent microsomal formation of Ph from N-hydroxy-Ph.