Pharmacological activity of the dinitrate metabolites of nitroglycerin following their oral administration to healthy volunteers

Organic nitrate metabolism and action: toward a unifying hypothesis and the future-a dedication to Professor Leslie Z. Benet

This review summarizes the major advances that had been reported since the outstanding contributions that Professor Benet and his group had made in the 1980s and 1990s concerning the metabolism and pharmacologic action of organic nitrates (ORNs). Several pivotal studies have now enhanced our understanding of the metabolism and the bioactivation of ORNs, resulting in the identification of a host of cysteine-containing enzymes that can carry out this function. Three isoforms of aldehyde dehydrogenase, all of which with active catalytic cysteine sites, are now known to metabolize, somewhat selectively, various members of the ORN family. The existence of a long-proposed but unstable thionitrate intermediate from ORN metabolism has now been experimentally observed. ORN-induced thiol oxidation in multiple proteins, called the "thionitrate oxidation hypothesis," can be used not only to explain the phenomenon of nitrate tolerance, but also the various consequences of chronic nitrate therapy, namely, rebound vasoconstriction, and increased morbidity and mortality. Thus, a unifying biochemical hypothesis can account for the myriad of pharmacological events resulting from nitrate therapy. Optimization of the future uses of ORN in cardiology and other diseases could benefit from further elaboration of this unifying hypothesis.

Clinical pharmacokinetics and pharmacodynamics of glyceryl trinitrate and its metabolites

This review discusses the pharmacokinetics and pharmacodynamics of glyceryl trinitrate (nitroglycerin; GTN) pertinent to clinical medicine. The pharmacokinetics of GTN associated with various dose regimens are characterised by prominent intra- and inter-individual variability. It is, nevertheless, important to clearly understand the pharmacokinetics and characteristics of GTN to optimise its use in clinical practice and, in particular, to obviate the development of tolerance. Measurements of plasma concentrations of GTN and of 1,2-glyceryl dinitrate (1,2-GDN), 1,3-glyceryl dinitrate (1,3-GDN), 1-glyceryl mononitrate (1-GMN), and 2-glyceryl mononitrate (2-GMN), its four main metabolites, remain difficult and require meticulous techniques to obtain reliable results. Since GDNs have an effect on haemodynamic function, pharmacokinetic analyses that include the parent drug as well as the metabolites are important. Although the precise mechanisms of GTN metabolism have not been elucidated, two main pathways have been proposed for its biotransformation. The first is a mechanism-based biotransformation pathway that produces nitric oxide (NO) and contributes directly to vasodilation. The second is a clearance-based biotransformation or detoxification pathway that produces inorganic nitrite anions (NO(2) -). NO(2) - has no apparent cardiovascular effect and is not converted to NO in pharmacologically relevant concentrations in vivo. In addition, several non-enzymatic and enzymatic systems are capable of metabolising GTN. This complex metabolism complicates considerably the evaluation of the pharmacokinetics and pharmacodynamics of GTN. Regardless of the route of administration, concentrations of the metabolites exceed those of the parent compound by several orders of magnitude. During continuous steady-state delivery of GTN, for instance by a patch, concentrations of 1,2-GDN are consistently 2-7 times higher than those of 1,3-GDN, and concentrations of 2-GMN are 4-8 times higher than those of 1-GMN. Concentrations of GDNs are approximately 10 times higher, and of GMNs approximately 100 times higher, than those of GTN during sustained administration. The development of tolerance is closely related to the metabolism of GTN, and can be broadly categorised as haemodynamic tolerance versus vascular tolerance. Efforts are warranted to circumvent the development of tolerance and facilitate the use of GTN in clinical practice. Although this remains to be accomplished, it is likely that, in the near future, regimens will be developed based on a full understanding of the pharmacokinetics and pharmacodynamics of GTN and its metabolites.

Opportunities for the treatment of erectile dysfunction by modulation of the NO axis--alternatives to sildenafil citrate

Erectile function in man depends upon a complex interaction of psychogenic, neurologic, hormonal and vascular factors, and therefore the management of erectile dysfunction (ED) reflects this complexity of control. Therapeutic options include psychological and non-pharmacological approaches as well as drug treatments. The effectiveness of the type-5 cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra) confirms the pivotal role of the NO-cGMP axis in promoting and maintaining erection. Although widely acclaimed, sildenafil leaves many questions unanswered, especially regarding its susceptibility to pharmacokinetic drug interactions, and its safety in patients with ischaemic heart disease and those taking nitrates. In view of the epidemiological link between erectile dysfunction and cardiovascular disease in the elderly, this limitation might have much broader implications. The presently available scientific documentation, although less extensive, indicates that NO donors, such as topically applied nitroglycerin (GTN; for example, 1-2 puffs of an ordinary GTN spray applied to the shaft of the penis), might be a reasonable alternative. Further larger-scale research on the efficacy and tolerability of topical GTN is needed to establish its full therapeutic potential in the treatment of erectile dysfunction.

Comparative pharmacokinetics and bioavailability of nitroglycerin and its metabolites from Transderm-Nitro, Nitrodisc, and Nitro-Dur II systems using a stable-isotope technique

The pharmacokinetics and bioavailability of nitroglycerin (GTN) and its metabolites, 1,2-glyceryl dinitrate (1,2-GDN) and 1,3-glyceryl dinitrate (1,3-GDN), were compared after a single 14-hour application of Transderm-Nitro (Ciba-Geigy, Summit, NJ), Nitrodisc (GD Searle, Chicago, IL), and Nitro-Dur II (Key Pharmaceuticals, Kenilworth, NJ) systems to 18 healthy male subjects on 3 separate occasions. A 14-hour intravenous infusion of 15N-labeled GTN was given simultaneously to correct for changes in systemic clearance during the application of each system. Plasma concentrations of 15N-labeled GTN, unlabeled GTN, and their corresponding dinitrate metabolites were measured using a gas chromatography/mass spectrometry method. Results showed that the plasma concentration profiles of nitroglycerin and its metabolites for the three systems were similar during and after system removal. Mean (SD) total amounts (AUCp x CLiv) of GTN transdermally available after adjustment for 15N-labeled GTN clearance were 5.3 (2.1), 5.3 (2.0), and 5.4 (2.6) mg for Transderm-Nitro, Nitrodisc, and Nitro-Dur II, respectively. Mean (SD) AUC values for 1,2-GDN were 44.6 (15.8), 44.3 (16.1), and 42.8 (19.3) ng.h/mL for the 3 systems. Corresponding AUC values for 1,3-GDN were 9.3 (2.9), 9.7 (2.9), and 8.7 (3.0) ng.h/mL. Statistical analysis of the log-transformed data based on 90% conventional confidence interval showed that all 3 systems delivered equivalent amounts of nitroglycerin into the systemic circulation. The AUC ratios for 1,3-GDN to GTN, but not 1,2-GDN to GTN, were statistically different for the intravenous and transdermal routes during all 3 system applications, indicating that the formation and metabolism of 1,3-GDN was dependent on route of administration.

Clinical pharmacological equivalence of a novel FCH-free GTN spray with low ethanol content vs a FCH-containing GTN spray

The overall therapeutic equivalence of a fluorochlorohydrocarbon (FCH)-free glyceryl trinitrate (GTN) pump spray with a low ethanol content (TL) was investigated relative to an FCH-containing GTN spray (Nitrolingual; R), in terms of: (1) pharmacokinetic bioavailability, (2) pharmacodynamic responses as assessed by digital plethysmography (DPG), and (3) clinical perception upon application. Pharmacokinetically, the time courses of the plasma concentrations of GTN and its dinitrate metabolites, 1,2- and 1,3-GDN, subsequent to the sublingual administration of 0.8 mg GTN showed somewhat lower bioavailability of GTN and its metabolites than to the reference. Pharmacodynamically, the changes in the DPG signals after the application of 0.8 mg GTN with TL were biostatistically equivalent with R (estimated ratio TL/R for the maximum decrease of the ratio between the systolic a wave and c incisure: 0.98; 90% CI: 0.84-1.14; and for the average decrease of the c: a ratio: 0.97; 90% CI: 0.80-1.16). The time of occurrence of the maximum effect of TL was not significantly different from that of R (estimated difference TL-R: -2.25 min; 95% CI:-9.5 min to 2 min). In contrast, after the administration of an FCH-free GTN spray with a higher ethanol content (TH, active control), the effect had a slightly earlier onset (TH-R: -6 min, 95% CI:-9.5 to -2 min) and there was a higher average response (TH/R: 1.12: 90% CI: 0.95 to 1.34). However, TH was consistently judged to cause an extremely unpleasant burning sensation in the mouth and thus was perceived as distinctly different from R. In contrast, TL was well tolerated and could not be distinguished from R.

Mechanisms of action of nitrates

Glyceryl trinitrate, isosorbide dinitrate, and isosorbide-5-mononitrate are organic nitrate esters commonly used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Organic nitrate esters have a direct relaxant effect on vascular smooth muscles, and the dilation of coronary vessels improves oxygen supply to the myocardium. The dilation of peripheral veins, and in higher doses peripheral arteries, reduces preload and afterload, and thereby lowers myocardial oxygen consumption. Inhibition of platelet aggregation is another effect that is probably of therapeutic value. Effects on the central nervous system and the myocardium have been shown but not scrutinized for therapeutic importance. Both the relaxing effect on vascular smooth muscle and the effect on platelets are considered to be due to a stimulation of soluble guanylate cyclase by nitric oxide derived from the organic nitrate ester molecule through metabolization catalyzed by enzymes such as glutathione S-transferase, cytochrome P-450, and possibly esterases. The cyclic GMP produced by the guanylate cyclase acts via cGMP-dependent protein kinase. Ultimately, through various processes, the protein kinase lowers intracellular calcium; an increased uptake to and a decreased release from intracellular stores seem to be particularly important.

Pharmacodynamic models of nitroglycerin-induced hemodynamic tolerance in experimental heart failure

Pharmacodynamic tolerance during continuous nitroglycerin (NTG) infusion is a significant limitation of nitrate therapy. The mechanism of this phenomenon is not well-understood but may involve physiologic compensation which involves vasoconstriction. We have obtained pharmacodynamic data on NTG-induced hemodynamic tolerance in a rat model of congestive heart failure (CHF), which we have shown to mimic human behavior toward NTG in vivo. In this report, we developed two mechanism-based pharmacokinetic/pharmacodynamic models to describe the time-dependent effects of NTG infusion on left ventricular end-diastolic pressures (LVEDP) in CHF rats and compared their abilities to describe the observed hemodynamic data. Both mathematical models introduced a counter-regulatory vasoconstrictive effect as a result of NTG-induced vasodilation and assumed the magnitude of this effect to be driven by the extent of the initial hemodynamic effect produced by NTG. The decay of this counter-regulatory effect was described by a first-order process in both models. A model that assumed vasoconstriction to develop via two sequential first-order processes was statistically superior in describing the data, when compared to one that assumed a single first-order process and a lag phase. Both models provided similar estimates of the half-life for the disappearance of the vasoconstriction (t1/2 of vasoconstriction: 128min vs. 182min, respectively), and both predicted rebound elevation of LVEDP after abrupt NTG withdrawal. These results are consistent with a counter-regulatory mechanism of NTG-induced hemodynamic tolerance and suggest that such an approach may be useful for modeling other tolerance phenomena as well.

Vascular and anti-platelet actions of 1,2- and 1,3-glyceryl dinitrate

1. The aim of this study was to investigate whether two metabolites of glyceryl trinitrate (GTN), 1,2 and 1,3-glyceryl dinitrate (1,2-GDN and 1,3-GDN) could account for the pharmacological effects of GTN. To this end the formation of nitric oxide (NO) from 1,2- and 1,3-GDN in the presence of bovine aortic smooth muscle cells (SMC) or endothelial cells (EC) was studied. The effects of various thiols on NO formation from these dinitrates was also evaluated. 2. 1,2-GDN or 1,3-GDN (10(-10)-10(-5) M) caused a dose-dependent relaxation of rabbit aortic strips denuded of endothelium and precontracted with phenylephrine. The dinitrates were less than one tenth as potent as GTN. 3. Incubation of 1,2-GDN or 1,3-GDN (75-2400 microM) with SMC for 30 min led to a concentration-dependent increase in nitrite (NO2-) formation but this increase was less than that produced from GTN. Likewise incubation of 1,2-GDN or 1,3-GDN with N-acetylcysteine (NAC), glutathione (GSH) or thiosalicylic acid (TSA) (all at 1 mM) for 30 min at 37 degrees C produced a concentration-dependent increase in NO2- formation. 4. Platelet aggregation induced by thrombin (40 mu ml-1) was not modified by high concentrations of 1,2-GDN or 1,3-GDN (175-700 microM). However, aggregation was inhibited when platelets were exposed to 1,2-GDN or 1,3-GDN (700 microM) in the presence of SMC (0.24-1.92 x 10(5) cells) or EC (0.8-3.2 x 10(5) cells). These effects were abrogated by co-incubation with oxyhaemoglobin (OxyHb, 10 microM) indicating that they were due to NO release. The concentrations of the dinitrates required to inhibit platelet aggregation by 50% were about 15 times higher than for GTN in the presence of the same numbers of SMC or EC.5. When NAC or TSA (both at 0.5 mM) were co-incubated with platelets for 3 min in the presence of1,2-GDN or 1,3-GDN, a concentration-dependent inhibition of platelet aggregation was observed. These anti-platelet effects were abolished by co-incubation with OxyHb (10 microM). Glutathione had no potentiating effects.6. Thus the dinitrate metabolites of GTN are metabolized to NO by SMC or EC and are acted upon by thiols to form NO at concentrations about 10 times higher than those of GTN. In vivo, after oral or intravenous GTN, GDN levels are reached which are more than 10 times higher than those of GTN.These data support the notion that part of the effects of GTN are due to the generation of NO from 1,2-GDN and 1,3-GDN by the cells of the vascular wall.

Pharmacokinetics and pharmacodynamics of nitroglycerin and its dinitrate metabolites in conscious dogs: intravenous infusion studies

Intravenous infusions of nitroglycerin (GTN), 1,2-glyceryl dinitrate (1,2-GDN), and 1,3-glyceryl dinitrate (1,3-GDN) were given to four conscious dogs at 10 micrograms/min, 30 micrograms/min, 50 micrograms/min, and 70 micrograms/min of GTN and 20 micrograms/min and 100 micrograms/min of GDNs. The steady state plasma concentrations (Css) of GTN were reached after about 60 min whereas for 1,2-GDN and 1,3-GDN the Css were reached at about 150 min after the infusion began. Except for one dog, the Css of GTN were not proportional to infusion rate, however, all dogs together showed a good linear relationship between Css of GTN and infusion rates with an average correlation coefficient of 0.917 +/- 0.102. Large variability in GTN clearance after various infusion rates was observed in all dogs. The Css ratios of 1,2-GDN/GTN and 1,3-GDN/GTN yield overall averages of 31.5 +/- 17.2 and 5.47 +/- 3.19, respectively. Average Css ratios of metabolites 1,2-GDN/1,3-GDN were 5.78 +/- 1.23. This ratio is different from those obtained after iv bolus and oral dosing indicating that the biotransformation of GTN to 1,2-GDN and 1,3-GDN differs for each dosing route. The clearances for 1,2-GDN and 1,3-GDN were not changed over the dose range of 20 micrograms/min to 100 micrograms/min. Terminal half-lives of 1,2-GDN and 1,3-GDN postinfusion were similar to those values obtained after a single bolus dose (45 min). It appears that all the GTN dose at steady state can be accounted for by the formation of measurable 1,2-GDN and 1,3-GDN. Large intra- and interdog variations in systolic blood pressure decrease (SPD) following infusions of GTN were observed, however, all dogs showed a clear systolic blood pressure decrease when the highest infusion rate (70 micrograms/min) was given. No significant systolic blood pressure drop was detected following 20 micrograms/min infusions of 1,2-GDN or 1,3-GDN. It was clear that systolic blood pressure in all dogs decreased following 100 microgram/min infusions of 1,2-GDN or 1,3-GDN. When SPD values were plotted vs. log GTN concentrations following the infusion of 70 micrograms/min of GTN in all four dogs, a counterclockwise hysteresis was observed indicating the significant contribution of the active dinitrate metabolites to GTN pharmacodynamics.

Simultaneous pharmacodynamic modeling of the non-steady-state effects of three oral doses of 1,3-glyceryl dinitrate upon blood pressure in healthy volunteers

The organic nitrate 1,3-glyceryl dinitrate (1,3-GDN) is one of the primary dinitrate metabolites of the antianginal agent nitroglycerin (GTN). Investigational New Drug Approval was sought to administer oral solution doses of 1,3-GDN to a small number (n = 3) of healthy volunteers; each subject receiving three doses at 1.2, 2.4, and 3.6 mg. With volunteers confined to a semirecumbent posture for the duration of each treatment (4-hr period postdose), diastolic blood pressure (DBP) was recorded and plasma samples collected for 1,3-GDN concentration analysis. Appreciable concentration-related decreases in DBP were observed, with maximal decreases from predose baseline values approximating 11 to 25 mm Hg. For each subject parametric pharmacodynamic modeling was performed with simultaneous analysis utilizing the DBP vs. time data from all three doses; an inhibitory Emax pharmacodynamic model was adopted. The temporal relationship between plasma 1,3-GDN concentrations and DBP displayed rapid equilibration. For subjects 1, 2 and 3, respectively, Emax was predicted as 12.9, 23.4, and 29.7 mm Hg, representing 21.5, 31.6, and 39.5% decreases in DBP from predose baseline values; plasma concentrations at half Emax (C50) were 2.75, 2.43, and 5.93 micrograms/L. Utilizing pharmacokinetic-pharmacodynamic modeling, 1,3-GDN plasma concentrations appear to relate to a systemic "effect measure" that is mechanistically representative of the therapeutic actions of organic nitrates as peripheral vasodilators. The establishment of a GDN plasma concentration-effect relationship together with the relatively high plasma levels of GDN achieved following GTN dosing supports the hypothesis that the GDNs contribute significantly to the hemodynamic effect observed with GTN.

Heating and cooling of the nitroglycerin patch application area modify the plasma level of nitroglycerin

19 healthy volunteers wore a nitroglycerin patch releasing 10 mg per 24 h for 2 h. Subsequently, the skin area surrounding the patch was exposed to 15 min of local heating with an infrared bulb (Group A, n = 10), or local cooling with an ice-pack (Group B, n = 9). The patch was protected by an insulating shield (Styrofoam). After 10 min of heating, the median (Walsh) plasma nitroglycerin level increased from 3.1 to 7.6 nmol.l-1. Body temperature remained constant. After 15 min of cooling the median plasma level had dropped from 2.1 to 1.4 nmol.l-1. The results demonstrate that changes in skin temperature may cause extensive short-term changes in the bioavailability of nitroglycerin. Presumably, a subcutaneous or cutaneous reservoir builds up during transdermal treatment, and changes in regional cutaneous blood flow affect the rate of drainage from the reservoir into the systemic circulation.

Dose proportionality of transdermal nitroglycerin

The FDA Cooperative Efficacy Study of transdermal nitroglycerin utilized a combination of marketed products over a wide dose range. Unfortunately, plasma nitroglycerin concentrations were not determined. The current study was conducted to assess plasma nitrate concentrations after transdermal doses of 15, 30, 60, and 105 mg/24 hr employing the FDA Cooperative Study design. Plasma concentrations of nitroglycerin, 1,3-glyceryl dinitrate, and 1,2-glyceryl dinitrate were determined during the 24 hr of application and for 1 hr after transdermal system removal. Dose proportionality was assessed after normalizing the data by theoretical dose. For nitroglycerin, dose-normalized AUC(0-infinity) and Cmax were higher for the 105 mg/24 hr dose than for the other doses. For the metabolites, 1,3-glyceryl dinitrate and 1,2-glyceryl dinitrate, there were no differences in dose-normalized AUC(0-infinity) and dose-normalized Cmax between the dose levels. No differences were seen in Tmax between the dose levels for all three species. Based on the dinitrate metabolites, dose proportionality was seen over the 15 to 105 mg/24 hr dose range. Nitroglycerin, however, was found to be linear only between 15 and 60 mg/24 hr.

Improved gas chromatographic assay for the simultaneous determination of nitroglycerin and its mono- and dinitrate metabolites

A sensitive, specific capillary gas chromatographic-electron-capture detection method for the simultaneous determination of nitroglycerin (GTN), 1,2- and 1,3-glyceryl dinitrate (1,2-GDN and 1,3-GDN, respectively) and 1- and 2-glyceryl mononitrate (1-GMN and 2-GMN, respectively) is reported. The minimum quantifiable concentration for GTN, GDNs and GMNs is 0.4 ng/ml in plasma, with extraction recoveries for GMNs greater than 76% and for GTN and the GDNs greater than 95%. Over the full range of quantifiable concentrations the inter-run assay precision and accuracy were less than 13 and 11%, respectively, for all five nitrates. Similar intra-run assay precision and accuracy values were found. The method was employed in the preliminary in vitro examination of GTN, GDN and GMN kinetics in human blood. Following addition of GTN to human blood, the ratio of 1,2-GDN to 1.3-GDN maximum concentrations (Cmax) was ca. 7:1, reflecting preferential denitration of the GTN molecule at the primary positions, while the Cmax ratio for 2-GMN to 1-GMN in this system was ca. 6:1, representing a highly selective if not specific primary denitration of the 1,2-GDN molecule. Following the intravenous administration of 1,2-GDN to five healthy male volunteers, 2-GMN/1-GMN Cmax ratios averaged 8.8:1, representing a highly selective but not specific formation of 2-GMN from the 1,2-GDN molecule. The assay will find utility in in vitro studies attempting to address the molecular pharmacology of GTN and its metabolites, and in in vivo clinical pharmacology studies attempting to address the relationship between pharmacokinetics and pharmacodynamics of GTN and its metabolites.

Percutaneous penetration kinetics of nitroglycerin and its dinitrate metabolites across hairless mouse skin in vitro

The percutaneous penetration kinetics of the antianginal, nitroglycerin (GTN), and its primary metabolites, 1,2- and 1,3-glyceryl dinitrate (1,2- and 1,3-GDN), were evaluated in vitro, using full-thickness hairless mouse skin. GTN and the 1,2- and 1,3-GDNs were applied (a) in aqueous solution as pH 7.4 phosphate-buffered saline (PBS) and (b) incorporated into lipophilic ointment formulations. The cutaneous transformation of GTN to its dinitrate metabolites was detected, but no interconversion between 1,2-GDN and 1,3-GDN was observed. Following application of the nitrates in PBS solution, all three compounds exhibited steady-state transport kinetics. The steady-state flux of GTN (8.9 +/- 1.5 nmol cm-2 hr-1) was significantly greater (P less than 0.05) than those of 1,2-GDN (0.81 +/- 0.54 nmol cm-2 hr-1) and 1,3-GDN (0.72 +/- 0.20 nmol cm-2 hr-1). The corresponding permeability coefficient (rho) for GTN (20 +/- 3 x 10(-3) cm hr-1) was significantly larger than the corresponding values for 1,2-GDN (1.4 +/- 0.9 x 10(-3) cm hr-1) and 1,3-GDN (1.2 +/- 0.4 x 10(-3) cm hr-1), which were statistically indistinguishable (P greater than 0.05). Further analysis of the transport data showed that the differences between GTN and the GDNs could be explained by the relative stratum corneum/water partition coefficient (Ks) values of the compounds. The apparent partition parameters, defined as kappa = Ks.h [where h is the diffusion path length through stratum corneum (SC)] were 19.8 +/- 2.5 x 10(-2) cm for GTN and 1.91 +/- 1.07 x 10(-2) and 1.81 +/- 0.91 x 10(-2) cm for 1,2- and 1,3-GDN, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)