Review of limited systemic absorption of orlistat, a lipase inhibitor, in healthy human volunteers

Heparin-induced Release of Protein-bound Solutes during Hemodialysis Is an in Vitro Artifact

Background: Several studies have pointed to a release of drugs or protein-bound solutes from their binding sites during heparinization. The effect is attributed to the metabolism of triglycerides to free fatty acids (FFAs), which compete with drugs for protein binding sites. This study evaluated the impact of intradialytic heparin on the free concentration of the uremic toxin p-cresol and on FFAs.

Methods: Blood samples from hemodialysis (HD) patients, before and during HD, were collected with selected anticoagulation strategies. We assessed the effects of standing time, temperature, pH, and the addition of a lipase inhibitor, tetrahydrolipstatin (THL) to blood samples on the free p-cresol concentration. p-Cresol was analyzed by HPLC with fluorescence detection. We measured FFAs by gas chromatography, and the free fractions of added valproic acid and phenytoin were evaluated by fluorescence polarization immunoassay.

Results: In blood samples (n = 22) not submitted to a specific treatment, free p-cresol increased from 9.9 ± 5.1 to 31.9 ± 22.3 μmol/L after 30 min of heparin HD (P <0.001) and correlated significantly with FFAs (r = 0.80; P = 0.002; n = 12). There was no increase in free p-cresol during heparin-free HD (n = 6) and trisodium citrate HD (n = 9). In addition, p-cresol in ultrafiltrates (n = 3) did not correspond to the free p-cresol in heparinized blood, suggesting that the increase in free p-cresol was artifactual. The release of p-cresol in the test tube was enhanced by standing time (n = 6), sample temperature (n = 6), and alkaline pH (n = 6). Inhibition of lipase activity with THL prevented the increase of FFAs (n = 6) and the release of free p-cresol during HD (n = 22). These results were corroborated by the study of the free fraction of valproic acid (n = 6) and phenytoin (n = 6).

Conclusions: The free concentrations of protein-bound solutes in plasma of heparinized patients are influenced by external factors that alter the lipase activity in the test tube. The free fraction does not increase during HD when lipase activity is neutralized at the time of blood sampling, so that previously reported increases are probably artifacts.

Orlistat--a novel weight loss therapy

OBJECTIVE

To review the pharmacology, pharmacokinetics, clinical safety and efficacy, drug interactions, and therapeutic issues related to the use of orlistat for treatment of obesity.

DATA SOURCES

English-language articles were identified from MEDLINE (1966-July 2000), Roche Laboratories, organizational guidelines, National Institutes of Health and Food and Drug Administration Web sites, and Doctor's Guide online. Key words included obesity, orlistat, and lipase inhibitors. References were also identified from reference sections of published articles.

STUDY SELECTION AND DATA EXTRACTION

Prospective, randomized, double-blind, placebo-controlled, human trials were selected for review and discussion.

DATA SYNTHESIS

Orlistat is the first agent in the lipase inhibitor class of antiobesity drugs. Orlistat is minimally absorbed and has been shown to reduce body weight by inhibiting absorption (by approximately 30%) of ingested dietary fat. Safety and efficacy have been established in one- and two-year double-blind, placebo-controlled trials; adverse effects were primarily, and almost exclusively, gastrointestinal. Due to its ability to block fat absorption, orlistat also has the capability to inhibit absorption of fat-soluble vitamins. Therefore, a daily multiple vitamin is recommended while taking orlistat.

CONCLUSIONS

By inhibiting fat absorption, orlistat offers a new treatment modality for weight loss and maintenance. Preliminary data from clinical trials suggest that orlistat may be beneficial in patients with comorbid conditions related to obesity, such as diabetes and hyperlipidemia. However, further studies during postmarketing surveillance are needed to fully establish orlistats long-term benefits and safety. Orlistat should be considered a useful adjunctive therapy for weight loss and maintenance in obese patients (i.e., body mass index > or = 30 kg/m2 or > or = 27 kg/m2 if other risk factors are present) committed to lifestyle changes including diet, exercise, and behavioral modification.

Orlistat, a new lipase inhibitor for the management of obesity

Orlistat, a weight-loss agent with a novel mechanism of action, recently was approved by the Food and Drug Administration for the treatment of obesity. It inhibits gastric and pancreatic lipases in the lumen of the gastrointestinal tract to decrease systemic absorption of dietary fat. In several trials lasting up to 2 years, orlistat was more effective than diet alone for weight reduction and maintenance of lost weight. Orlistat treatment also results in modest improvements in total cholesterol, low-density lipoprotein, blood pressure, and fasting glucose and insulin concentrations. The major adverse effects are gastrointestinal, usually occur early in therapy, and tend to decrease with continued treatment. Because orlistat may decrease the absorption of fat-soluble vitamins, a standard multiple-vitamin supplement is recommended daily during therapy to prevent abnormalities in vitamin serum concentrations. The potential for severe gastrointestinal discomfort and the modest degree of weight loss may limit the agent's clinical utility. Its long-term safety and effectiveness for weight maintenance, cost-effectiveness of treatment, and overall reduction in obesity-related morbidity and mortality remain to be determined.

Orlistat fails to alter postprandial plasma lipid excursions or plasma lipases in normal-weight male volunteers

OBJECTIVES

After 10 d of orlistat administration (120 mg three times/day), the primary objective was to determine the drug's effect on postprandial plasma lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) activities on day 10 after an oral fat-load. The secondary objectives were to determine the effects of orlistat on 12 h postprandial measures of: (1) preheparin HTGL and LPL; and (2) serum triglycerides, very-low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol and free fatty acids.

METHODS

Twenty-four normal-weight, healthy male volunteers were randomized to either 120 mg orlistat (n=12) or placebo (n=12) three times a day with meals for 10 d. Preheparin LPL and HTGL activities and LPL specific activity were measured in the fasted state on days 1, 5, and 10. On days 5 and 10 the study medication (orlistat or placebo) was taken at the beginning of a fat-rich breakfast and serum lipid and lipoprotein levels monitored for 12 h postprandially. On day 10, 15 min postheparin HTGL activity was measured 8 h after the fat-rich breakfast.

RESULTS

No differences were found between groups in fasting levels of preheparin LPL or HTGL activity or in LPL-specific activity on days 1, 5 and 10. No difference was found between the two treatment groups in postheparin HTGL activity 8 h after the fat-rich breakfast. Also, no differences were found between the two groups in plasma triglycerides or lipoproteins.

CONCLUSION

The results indicate that the oral administration of orlistat (120 mg t. i.d.) does not significantly alter plasma triglycerides or lipoproteins, and that the inhibitory effect of orlistat on lipases is limited to the gastrointestinal tract and is not manifested systemically.

Insulin resistance, impaired glucose tolerance and non-insulin-dependent diabetes, pathologic mechanisms and treatment: current status and therapeutic possibilities

Impaired glucose tolerance and non-insulin-dependent diabetes (NIDDM) are the pathologic consequence of two co-incident and interacting conditions, namely insulin resistance and relative insulin deficiency. Recognised by the World Health Authority as a global health problem there are at 1995 estimates at least 110 million diagnosed diabetics world wide with at least the same number undiagnosed. Diabetes is the 4th leading cause of death in developed countries and its management exerts a vast economic and social burden. Insulin resistance is established as the characteristic pathologic feature of patients with glucose intolerance and NIDDM describing a state in which insulin stimulated glucose uptake and utilisation in liver, skeletal muscle and adipose tissue is impaired and coupled to impaired suppression of hepatic glucose output. Although the biochemical mechanisms underpinning both defects are becoming better understood, the genetic and molecular causes remain elusive; and whether insulin resistance or relative insulin deficiency represents the primary defect in patients with NIDDM is the matter of some debate. In this article we review the biochemical and molecular nature of the defects in insulin sensitivity and glucose uptake, and discuss some of the potential causative mechanisms. The genetic and environmental basis of insulin resistance is reviewed and presented, and potential therapeutic targets including thiazolidinediones are discussed.

Long-term systemic exposure of orlistat, a lipase inhibitor, and its metabolites in obese patients

Orlistat, a lipase inhibitor, acts locally in the gastrointestinal tract; its systemic exposure is not required for its efficacy. However, knowledge of the extent of its systemic exposure is important for its safe use in obese patients, the intended target population. Pharmacokinetic screening in obese patients was carried out by monitoring plasma concentrations of unchanged orlistat and its metabolites in five key double-blind, placebo-controlled phase II/III studies. Results of these studies involving the monitoring of plasma samples indicate that detection of intact orlistat in plasma was sporadic, and measurable concentrations were low (< 10 ng/mL or 0.02 microM) without evidence of accumulation, which is consistent with minimal absorption. It is concluded that systemic exposure of orlistat is negligible; at a clinically efficacious dose level, orlistat is unlikely to produce systemic lipase inhibition.

Obesity medications and the treatment of type 2 diabetes

Obesity is a chronic disease and requires ongoing treatment. Type 2 diabetes is associated with obesity and improves with weight loss. Diets of 800 kcal/d induce twice the weight loss induced by weight loss medications. The strength of weight loss medication, which should be used with diet and a lifestyle change program, is the maintenance of weight loss. Sibutramine and orlistat are the only two medications approved for the long-term treatment of obesity. Orlistat gives a reduction of low-density lipoprotein (LDL) cholesterol in excess of that expected with weight loss, and the drop in blood pressure expected with weight loss is not seen with sibutramine. Except in newly diagnosed patients with diabetes subjects, patients with diabetes lose half the weight of subjects who do not have diabetes when treated with weight loss medications. Metformin and, to a lesser extent, acarbose cause weight loss, making them attractive choices for the treatment of obese type 2 diabetic subjects. Repaglinide appears to be weight-neutral, but other medications for patients with diabetes can be associated with weight gain. Many new medications are in development for the treatment of obesity. These new medications act through a variety of mechanisms and will surely play an increasingly important role in the treatment of obese patients with type 2 diabetes.

Orlistat

Orlistat (tetrahydrolipstatin) is an inhibitor of pancreatic and other lipases. As a pancreatic lipase inhibitor, it acts in the gastrointestinal lumen and is indicated for use in obesity. Serum total cholesterol and low density lipoprotein-cholesterol levels were reduced in obese, but otherwise healthy, patients during < or = 2 years' orlistat treatment; serum triglyceride and high density and very low density lipoprotein-cholesterol levels were unchanged in trials of < or = 12 weeks. Obese patients who were maintained on a hypocaloric diet and who received orlistat 360 mg/day for 12 weeks lost a significantly greater percentage of bodyweight than placebo recipients (5 vs 3.5%). In 2-year studies, weight loss was significantly greater in orlistat than in placebo recipients by the end of year 1; weight was further reduced or maintained in the second year, when a eucaloric diet was allowed, in orlistat but not placebo recipients. A greater proportion of orlistat than placebo recipients lost > 5% or > 10% of their initial bodyweight in 1- and 2-year studies.

Quantitative liquid chromatographic-tandem mass spectrometric determination of orlistat in plasma with a quadrupole ion trap

This report evaluates the use of a quadrupolar ion trap for quantitation in a bioanalytical laboratory. The evaluation was accomplished with the cross-validation of an LC-MS-MS quantitative method previously validated on a triple quadrupole mass spectrometer. The method was a multi-level determination of the anti-obesity drug, orlistat, in human plasma. The method has been refined previously on a triple quadrupole instrument to provide rapid sample throughput with robust reproducibility at sub-nanogram detection limits. Optimization of the method on the ion trap required improved chromatographic separation of orlistat from interfering plasma matrix components coextracted during the initial liquid-liquid extraction of plasma samples. The ion trap produces full-scan collision-induced dissociation mass spectra containing characteristic orlistat fragment ions that are useful for quantitation. Data collection on the ion trap required a precursor ion isolation width of 3.0 Da and optimal quantitative results were obtained when three fragment ions were monitored with a 1.8 Da window for each ion. Although a direct cross-validation between the ion trap and the tandem triple quadrupole mass spectrometer was not possible, quantitative results for orlistat comparable to those obtained from the triple quadrupole instrument were achieved by the ion trap with the modified method. The limit of quantitation for orlistat in plasma on the ion trap was 0.3 ng ml(-1) with a linear dynamic range of 0.3 to 10 ng ml(-1). Precision and accuracy varied from 4 to 15% over the quantitation range. The overall results provide an example of the utility of an ion trap in bioanalytical work.

Metabolic profiles of minimally absorbed orlistat in obese/overweight volunteers

To determine the metabolic profile of minimally absorbed orlistat in obese/overweight patients, an open-label, single-dose study was performed in eight obese/overweight volunteers between 23 and 68 years of age. Each subject received a single oral dose of 360 mg orlistat containing approximately 400 muCi of 14C-labeled orlistat. Serial blood samples were collected at specified times over 10 hours after administration of orlistat for determination of total radioactivity, unchanged orlistat, and major metabolites in the plasma. Urine samples were collected over 24 hours and analyzed to evaluate the urinary recovery of total radioactivity and the profile of orlistat metabolites in the urine. In addition, all fecal samples were collected and analyzed for total radioactivity. Urinary and fecal recovery of the administered dose of total radioactivity were 1.13 +/- 0.50% (24-hour data only) and 96.4 +/- 18.1% (n = 7), respectively. Maximum observed concentration (Cmax) and time to Cmax (tmax) values of plasma total radioactivity were 150 +/- 51 ng.eq/mL and 6.8 +/- 1.5 hrs, respectively. All these parameters obtained in obese/ overweight subjects were similar to those reported previously in healthy subjects. On the basis of the area under the concentration-time curve from 0 to 10 hours (AUC0-10), two major metabolites comprise a total of approximately 42% of the total radioactivity in plasma. The primary metabolite (M1) has a short half-life (approximately 2 hours), whereas the secondary metabolite (M3) disappeared at a slower rate. No strikingly apparent difference in the urinary metabolic profile was observed between two gender groups. It is concluded that the disposition of orlistat appears to be similar between normal and obese/overweight subjects. Of the minimal fraction of the dose that was absorbed systemically, the presence of two major metabolites accounts for approximately 42%.

The effect of orlistat on the pharmacokinetics of phenytoin in healthy volunteers

To assess the effect of an orlistat-induced reduction in dietary fat absorption on the pharmacokinetics of phenytoin, a third-party blind, placebo-controlled, randomized, two-way crossover study was performed in 12 healthy volunteers. Each participant received single 300-mg oral doses of phenytoin administered on the fourth day of treatment with 120 mg orlistat (treatment A) or placebo (treatment B) three times a day for 7 days. The two treatments were separated by a 2-week washout period. Serial blood samples were collected before and up to 96 hours after each dose of phenytoin to determine serum concentrations of phenytoin. The 90% confidence intervals (CI) for the ratio of geometric least-squares means for maximum concentration (Cmax) and area under the concentration-time curve (AUC) and for the difference of arithmetic least-squares means for time of maximum concentration (tmax) and elimination rate constant (lambda z) showed the two treatments of phenytoin to be equal by analysis of variance. An approximately 30% reduction in dietary fat absorption induced by orlistat administered at doses of 120 mg three times daily did not significantly alter the pharmacokinetics of a single 300-mg oral dose of phenytoin in healthy volunteers.

The effect of orlistat on the pharmacokinetics and pharmacodynamics of warfarin in healthy volunteers

To assess the effect of orlistat on the pharmacokinetics and pharmacodynamics of warfarin, a third-party blind, placebo-controlled, randomized, two-way crossover study was performed in 12 healthy volunteers. Each participant received single 30-mg oral doses of racemic warfarin sodium (Coumadin; DuPont Pharma, Wilmington, DE) administered on the eleventh day of treatment with 120 mg orlistat (treatment A) and placebo (treatment B) three times a day for 16 days; the two treatments were separated by a 3-week washout period. Serial blood samples were collected before and at appropriate intervals after each dose of warfarin to determine plasma concentrations of R-warfarin and S-warfarin and blood prothrombin time (PT) and plasma Factor VII concentration. In addition, serum concentrations of vitamin K1 and its epoxide and of osteocalcin and its undercarboxylated form were measured before breakfast on days -7, 1, 4, 6, and 10. Equivalent results between treatments with orlistat and placebo were found with regard to all pharmacokinetic parameters of R- and S-warfarin (except for time to maximum concentration of R-warfarin). Pharmacodynamic parameters of warfarin (PT and Factor VII) and vitamin K nutritional status parameters (ratios of vitamin K1 to vitamin K1 epoxide and undercarboxylated osteocalcin to osteocalcin) also were unaltered by orlistat. Orlistat administered at doses of 120 mg three times daily did not significantly alter the pharmacokinetics and pharmacodynamics of a single 30-mg oral dose of warfarin in healthy volunteers.

Lack of effect of orlistat on the bioavailability of a single dose of nifedipine extended-release tablets (Procardia XL) in healthy volunteers

Orlistat, a lipase inhibitor, reduces dietary fat absorption, and thus could potentially alter the absorption of some concomitantly administered drugs, such as the nifedipine gastrointestinal therapeutic system (GITS). To assess the effect of orlistat on the bioavailability of nifedipine GITS, a third party-blind, placebo-controlled, randomized, two-way crossover study was performed in 18 healthy volunteers. Each participant received single 60-mg oral doses of nifedipine GITS (Procardia XL; Pfizer Labs, New York, NY) on the fourth day of treatment with 120 mg of orlistat or placebo three times a day for 6 days. The two treatments were separated by a washout period of at least 1 week. Serial blood samples were collected before and at appropriate intervals after each nifedipine dose to determine plasma concentrations of nifedipine. The 90% confidence intervals for the ratio of geometric least-square means for maximum concentration (C(max)) and area under the concentration-time curve (AUCo-t) and for the difference of arithmetic least-square means for time to maximum concentration (t(max)) indicate that the bioavailability of nifedipine was not altered by treatment with orlistat. Therapeutic doses of 120 mg of orlistat three times daily do not significantly alter the bioavailability of a single 60-mg oral dose of nifedipine GITS in healthy volunteers.

The effect of orlistat, an inhibitor of dietary fat absorption, on the pharmacokinetics of beta-carotene in healthy volunteers

To assess the influence of orlistat, a lipase inhibitor, on the absorption of beta-carotene, an open-label, parallel, placebo-controlled, randomized, two-way crossover study was performed in 48 healthy volunteers between the ages of 19 and 58 years. Each subject received a single oral dose of 0, 30, 60, or 120 mg beta-carotene (12 subjects per dose level) on the fourth day of treatment with orlistat (120 mg) or placebo 3 times a day for 6 days. The treatments were separated by a washout period of at least 5 weeks. Serial blood samples were collected before and at appropriate intervals after administration of beta-carotene to determine plasma concentrations of unchanged beta-carotene. Short-term (3 to 6 days) treatment with orlistat did not alter endogenous profiles of beta-carotene in plasma. When beta-carotene was given during orlistat treatment, its absorption was reduced by approximately one-third. This reduction was consistent for all three dose levels of beta-carotene studied; however, the results for the 30-mg dose level were subject to greater variability, particularly for area under the concentration-time curve (AUC). It was concluded that two thirds of a supplemental dose of beta-carotene will be absorbed during orlistat treatment; this may be sufficient to achieve physiologic levels of beta-carotene with an appropriate dose of beta-carotene, should supplementation be needed in obese patients who have developed beta-carotene deficiency during therapy with orlistat.