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H L Fung

Publications and source records attributed to H L Fung.

At least 55 records · Page 3Linked to original sources
PubMed1994

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.

Animals↗
PubMed1994

A modified product inhibition model describes the nonlinear pharmacokinetics of nicorandil in rats.

Nicorandil, a vasodilator which acts through both cyclic GMP accumulation and K+ channel opening, has been used in the treatment of various cardiovascular diseases. We have examined the pharmacokinetics of nicorandil in the rat as a function of dose, as both i.v. boluses (9 doses, 0.75-12 mg, n = 1.4 per dose), and as a 5-hr infusion followed by a 5-hr washout (6 doses, 10-500 micrograms/kg/min, n = 3 per dose). Plasma nicorandil concentrations were determined by HPLC. Nicorandil plasma concentrations increased disproportionately with dose, but nicorandil elimination obeyed apparent monoexponential kinetics, and the apparent half-life (t1/2) increased with dose. In addition, the approach to apparent steady-state during the infusion phase was not overtly sensitive to the drastic changes in t1/2 observed. Pharmacokinetic modelling with several nonlinear models, viz: Michaelis-Menten with parallel first-order, cosubstrate depletion and competitive product inhibition, were carried out. Addition of the sulfhydryl donor, N-acetyl-L-cysteine, did not change the pharmacokinetics of nicorandil, providing experimental indication that a cosubstrate depletion model might not be applicable. To described the unique pharmacokinetics, a modified product inhibition model was developed. This new model includes the classic competitive product inhibition equation, describing both parent and product kinetics, and it incorporates, in addition, separate first-order elimination rate constants for both nicorandil and the inhibition metabolite. Experimental evidence showed that N-(2-hydroxyethyl) nicotinamide, the major metabolite of nicorandil in rats, and nicotinamide (niacinamide) itself, indeed inhibited nicorandil elimination.

Animals↗
PubMed1994

Pharmacokinetics, plasma protein binding and urinary excretion of N omega-nitro-L-arginine in rats.

The in vivo disposition of N omega-nitro-L-arginine (L-NOARG) has been examined in rats. Plasma concentrations of L-NOARG following an intravenous dose of 10 mg kg-1 were determined by high-performance liquid chromatography. Plasma L-NOARG concentrations declined biexponentially, with average half-lives of 11 min and 20 h. L-NOARG clearance did not appear to exhibit concentration-dependency below a plasma concentration of 4.56 x 10(-4) M (100 mg l-1). Ultrafiltration studies revealed insignificant binding of L-NOARG to rat plasma proteins. Urinary excretion of unchanged L-NOARG was minimal. These pharmacokinetic information may be useful in the design of in vivo experiments involving L-NOARG, as well as in the interpretation of the pharmacodynamics of this important nitric oxide synthase inhibitor.

Animals↗
PubMed1994

Utilization of antidrug antibody fragments for the optimization of intraperitoneal drug therapy: studies using digoxin as a model drug.

The direct administration of chemotherapeutic agents into the peritoneal cavity has been investigated as a method to treat cancers residing within the peritoneum. The benefits of i.p. drug administration are limited, however, by the systemic toxicity of antineoplastic drugs which diffuse out of the peritoneum and into the general circulation. We propose that antidrug antibody fragments may be useful in binding chemotherapeutics in the general circulation, thereby reducing the systemic tissue exposure and toxicity resulting from such i.p. therapy. Inasmuch as antibody fragments directed against antineoplastic agents are not available, we tested our hypothesis by using i.v. administered ovine antidigoxin Fab fragments and determined their ability to limit digoxin tissue exposure and toxicity in mice after an i.p. digoxin injection. The rate of digoxin disappearance from the peritoneal cavity and the fraction of digoxin unbound in the peritoneal cavity were also assessed to determine the effect of the antibody fragments on peritoneal exposure. Our results showed that the antidigoxin antibody fragments can greatly decrease digoxin tissue exposure and toxicity without affecting peritoneal exposure, unbound fraction of digoxin in the peritoneum or peritoneal digoxin disappearance rate. Although the utility of drug-binding antibodies and antibody fragments for the treatment of drug intoxication is well known, these results demonstrated the potential ability of antidrug antibody fragments to improve the site-specificity of drug therapy.

Animals↗
PubMed1993

Biochemical characterization of a membrane-bound enzyme responsible for generating nitric oxide from nitroglycerin in vascular smooth muscle cells.

A membrane-bound enzyme responsible for generating nitric oxide (NO) from nitroglycerin (NTG) in vascular smooth muscle cells has been characterized. The enzyme could be solubilized from vascular microsomes by several detergents, the most effective of which was 3-[(3-cholamidopropyl)-dimethylamino]-1-propanesulfonate (CHAPS). A partially purified enzyme preparation was obtained with CHAPS-solubilized vascular microsomes that were processed sequentially through an ion exchange column and a gel filtration column. The activity of this partially purified enzyme showed a dependence on substrate concentration, protein concentration and the duration of incubation. Enzyme activity was enhanced 2.7- to 4.2-fold by several thiols such as cysteine, N-acetylcysteine, reduced glutathione, and dithiothreitol. On the other hand, N-ethylmaleimide, iodoacetic acid, p-chloromercuric benzoic acid and 1-chloro-2,4-dinitrobenzene, reagents known to bind with the free sulfhydryl groups, inactivated the NO-generating activity from NTG. The enzyme activity could be reversibly bound to an organomercurial column. These results suggested the presence of a free thiol group in the enzyme and that this thiol group was required for enzyme activity. The partially purified enzyme was active in the presence of 0.1% sodium dodecyl sulfate (SDS). The enzyme was purified to near homogeneity using several sequential chromatographic steps including DEAE-Sephacel, Biogel A 1.5 m, hydroxylapatite and organomercurial columns, resulting in an increase in enzyme activity of about 94-fold. The subunit of this enzyme, as identified on an SDS-treated electrophoresis gel, had an apparent molecular size of 58 kDa.

Animals↗
PubMed1993

High-performance liquid chromatographic method for stability and pharmacokinetic studies of nicorandil.

Nicorandil is a unique vasodilator that combines the actions of a potassium channel activator and a nitrovasodilator. Little literature is available on its chemical stability and pharmacokinetics in animals. We developed a simple, specific, sensitive, and precise high-performance liquid chromatographic method for the determination of nicorandil in aqueous solution and rat plasma, achieving a detection limit of 0.3 microgram/ml with 180-microliters samples. Nicorandil was found to be relatively stable between pH 2 and 10 at 37 degrees C (half-life = 461-84 h) and it is subjected to specific base catalysis above pH 10. Both chemical degradation and in vivo metabolism produced N-(2-hydroxyethyl)nicotinamide, the denitrated product. Preliminary pharmacokinetic investigations showed that the assay is capable of quantitating nicorandil in rat plasma over a range of 0.3-100 micrograms/ml. These studies also suggested that the pharmacokinetics of nicorandil are dose-dependent.

Animals↗
PubMed1993

Clinical pharmacology of organic nitrates.

Although organic nitrates have been used in cardiovascular therapy for many years, various aspects of their pharmacology remain poorly understood. It is now known that organic nitrates produce nitric oxide (NO) in vascular smooth muscle cells, catalyzed by a membrane-bound enzyme that is not glutathione-S-transferase. Other nitrovasodilators, such as organic nitrites, sodium nitroprusside, and S-nitrosothiols, do not utilize the same enzyme for NO generation. The short-term hemodynamic action of various organic nitrates has been shown to be related to their pharmacokinetics, but their long-term therapeutic effects are limited by the development of pharmacologic tolerance. Nitrate sensitivity in patients can be restored daily after a nitrate-free period of 8-12 hours. Coadministration of nitrates with other vasodilators, such as captopril and hydralazine, may avoid the development of nitrate tolerance in patients with congestive heart failure.

Humans↗
PubMed1993

Relationship between nitroglycerin-induced vascular relaxation and nitric oxide production. Probes with inhibitors and tolerance development.

We have shown previously that nitric oxide (NO) is generated from nitroglycerin (NTG) through enzyme-mediated reactions in the bovine coronary artery smooth muscle cell, but it is not known whether this metabolic conversion plays a significant role in the pharmacologic action of NTG, viz. relaxation. In this study, we developed a technique that allowed direct measurement of NO from intact bovine coronary arterial rings that were incubated previously with NTG, and examined whether changes in NTG-induced relaxation were accompanied by parallel changes in NO generation. Co-incubation of the vascular preparations with a potent inhibitor of glutathione-S-transferases (GSTs), bromosulfophthalein (up to 200 microM), did not affect NTG-induced relaxation, nor did it alter NO generation from NTG in the preparation. In contrast, 1-chloro-2,4-dinitrobenzene (CDNB), a GST substrate, inhibited NO generation as well as the relaxation response of NTG in the intact vascular tissue preparation. CDNB, however, did not decrease the relaxant responses of nifedipine and isoproterenol. Thus, the inhibitory effect of CDNB on NTG-induced relaxation and NO production appeared specific. When bovine coronary rings were made tolerant to NTG by pretreatment with 0.44 mM NTG for 1 hr, the EC50 was shifted to the right 162-fold, and NO generation was also reduced in intact rings and tissue homogenates. However, when the homogenates were further subfractionated to microsomes and cytosols, or when homogenates were allowed to stand for a similar time period necessary for subfractionation, the difference in NO production from control versus tolerant tissue preparations disappeared. It is possible, therefore, that the NTG-induced tolerance process might have been partially reversed during this time period. Results of this study identified CDNB as an apparently specific inhibitor of NTG action, but showed that GST-mediated reactions were probably not involved in the metabolic activation of NTG. Our results also indicated that tissue NO generation from NTG was positively related to the relaxation responses generated by this nitrovasodilator.

Animals↗
PubMed1993

Pharmacokinetics of oral L-isoidide mononitrate in rats.

L-isoidide mononitrate (L-IIMN) is the most potent mononitrate vasodilator described so far in the literature. Since other mononitrates, such as isosorbide-5-mononitrate and isosorbide-2-mononitrate, have been shown not to be subject to first-pass metabolism, we examined the pharmacokinetics of L-IIMN after oral administration to determine whether this compound also exhibited this behavior. An oral dose of 2 mg kg-1 L-IIMN dissolved in normal saline was given to seven rats. Absorption of L-IIMN after dosing was rapid with an apparent absorption half-life of 9.5 +/- 3.6 min (mean +/- SD). Plasma L-IIMN concentrations peaked between 5 and 20 min after dosing and declined thereafter in an apparently monoexponential manner. The average elimination half-life was 11.9 +/- 1.7 min (mean +/- SD). Oral bioavailability was estimated to be about 50%. Thus, unlike the other mononitrates so far examined in the literature, L-IIMN exhibits incomplete bioavailability. This pharmacokinetic behavior, however, is consistent with its faster systemic clearance compared to other organic mononitrates.

Administration, Oral↗
PubMed1993

Pharmacokinetics of stereomeric 1,4:3,6-dianhydrohexitol mononitrates in rats.

The pharmacokinetics and urinary recoveries of four isomeric mononitrates, L-isoidide mononitrate (L-IIMN), isosorbide-2-mononitrate (IS-2-MN), isomannide mononitrate (IMMN), and isosorbide-5-mononitrate (IS-5-MN), were investigated at an intravenous dose of 2 mg/kg in rats. All four compounds exhibited monoexponential kinetics at this dose. The volumes of distribution were similar for all four isomers and were estimated at about 1.0 liter/kg. The systemic clearances of L-IIMN, IMMN, IS-2-MN, and IS-5-MN were 65.1 +/- 13.0, 32.7 +/- 12.0, 11.0 +/- 2.3, and 8.23 +/- 1.82 ml/min/kg, respectively (P < 0.05, all pairwise comparisons). Free mononitrate in the urine accounted for 0.306 to 4.56% of the administered dose, while the recovery in conjugated forms (after glusulase hydrolysis) accounted for 42.8% of the IMMN dose and 7.70 to 14.5% of the dose of the remaining three isomers. The dose-dependent pharmacokinetics of three of the mononitrates were explored at selected higher doses which cause equivalent vasodilator responses, L-IIMN (22 mg/kg), IS-2-MN (100 mg/kg), and IS-5-MN (300 mg/kg). The clearances of L-IIMN, IS-2-MN, and IS-5-MN at these higher doses were 42.3 +/- 5.7, 6.38 +/- 0.59, and 3.33 +/- 0.62 ml/min/kg, respectively, all significantly less than those found at the 2 mg/kg dose. Typical Michaelis-Menten-type curvatures were observed in the concentration-time curves after IS-2-MN and IS-5-MN dosing. The pharmacokinetics of L-IIMN were also dose dependent, but they could not be described by simple Michaelis-Menten kinetics.

Animals↗
PubMed1993

Effect of apparent elimination half-life on nitroglycerin-induced hemodynamic rebound in experimental heart failure.

Hemodynamic rebound after abrupt withdrawal may be an important consideration associated with nitroglycerin (NTG) monotherapy. This phenomenon may arise from unopposed neurohormonal vasoconstriction because of rapid elimination of NTG. The role of NTG pharmacokinetics in the development of hemodynamic rebound was examined using a rat model of congestive heart failure. NTG was infused for 90 min, then the dose was either abruptly stopped (n = 8) or gradually reduced by 20% every 20 min (n = 7). Abrupt withdrawal caused rebound elevations of left ventricular end-diastolic pressure (LVEDP) to about 25% above baseline values, at 30-60 min after drug termination (P < 0.01), but this was completely avoided by graded NTG withdrawal. A positive correlation was observed (P < 0.05) between the percentage reduction in LVEDP during infusion and the maximum percentage rebound in rats after abrupt withdrawal but not after graded withdrawal. These results suggest that NTG-induced hemodynamic rebound is related to its short biological half-life and that this phenomenon is consistent with a mechanism of neurohormonal compensation.

Animals↗
PubMed1993

Hemodynamic tolerance and pharmacokinetics of nicorandil in experimental heart failure.

We tested whether nicorandil (NCR), a nitrovasodilator which also possesses K+ channel action, will produce in vivo hemodynamic tolerance commonly observed with other nitrovasodilators such as nitroglycerin (NTG). NCR was infused at 50 micrograms/kg/min for 10 h through a femoral cannula into conscious rats (n = 5) with congestive heart failure (CHF). Left ventricular end-diastolic pressure (LVEDP), LV peak-systolic pressure (LVPSP), and heart rate (HR) were measured periodically through an implanted catheter. Under these conditions, NCR produced a 40% reduction (p < 0.05) in LVEDP from baseline at 40 min after the start of the infusion. This reduction lasted approximately 6 h, followed by a slow return to baseline between 6 and 10 h. LVPSP was reduced (p < 0.05) from 2 h and leveled at approximately 8 h. HR increased by 12% from baseline at 40 min (p < 0.05). Plasma NCR concentrations were identical at 5 h (pretolerant state) and at 10 h (tolerant state) in these animals, suggesting that plasma pharmacokinetics were unrelated to NCR pharmacodynamics. NCR kinetics were not changed by the presence of CHF. Thus, in this animal model, tolerance to NCR developed in LVEDP but not LVPSP, and the K+ channel action of NCR apparently could not prevent tolerance development to venodilation arising from nitrate action.

Animals↗
PubMed1993

Continuous versus intermittent nitroglycerin administration in experimental heart failure: vascular relaxation and radioligand binding to adrenoceptors and ion channels.

Continuous nitroglycerin (NTG) administration causes pharmacologic tolerance in humans and animals, whereas intermittent dosing is capable of avoiding or reducing tolerance development. The mechanism of NTG-induced hemodynamic tolerance may involve specific vascular desensitization and/or neurohormonal compensation. We compared effects of long-term (10 days) NTG administration (continuous or intermittent 12 h on/12 h off transdermal dosing, 10 micrograms/min) to rats with congestive heart failure (CHF) on radioligand binding from selected tissues. Tension responses in isolated blood vessels, plasma renin activity (PRA), plasma Na+ and K+ concentrations were also determined. The maximal binding values (Bmax) for [3H]glyburide and [3H]PN 200 110 in homogenates of left ventricle, right ventricle, and brain were not significantly different after NTG administration (continuous or intermittent), as compared with control. Intermittent, but not continuous, NTG caused significant increases in beta-adrenoceptor densities in the left ventricle, as judged by [3H]dihydroalprenolol binding (Bmax values: intermittent NTG 34.5 +/- 4.8, continuous NTG 24.4 +/- 2.6, placebo control 20.9 +/- 2.9 fmol/mg protein); Kd values for all ligands were not significantly altered by NTG administration. Both intermittent and continuous NTG increased the vascular contractile response to phenylephrine in isolated rat thoracic aorta. Slight reductions (two- to four-fold shifts in EC50 values) in thoracic aorta relaxant response to NTG were observed in both treatment groups as compared with control. Intermittent and continuous NTG administration caused selective changes in beta-adrenoceptor density and vascular response. These changes may contribute partly to the phenomenon of pharmacologic tolerance after chronic nitrate administration.

Administration, Cutaneous↗
PubMed1992

Biochemical mechanism of organic nitrate action.

Increasing evidence suggests that organic nitrate action derives from their metabolic conversion to nitric oxide (NO) in the vascular smooth muscle cell. The primary catalytic activity of this process appears to reside at the cellular plasma membrane. There is no concrete evidence to indicate that NO formation is preceded by the production of inorganic nitrite ion or that the NO produced needs to form S-nitrosothiols before it can activate guanylate cyclase to produce cyclic guanosine 3',5'-monophosphate (cGMP). Although sulfhydryl donors can partially reverse nitroglycerin-induced tolerance in patients, this phenomenon (by itself) is not sufficient to implicate intracellular sulfhydryl depletion as an operating mechanism of clinical nitrate tolerance. This is because sulfhydryl donors can react with nitroglycerin extracellularly to form S-nitrosothiols, and nonsulfhydryl compounds, such as enalapril and hydralazine, can prevent the development of in vivo nitrate tolerance. In addition to the cellular biochemical reactions, organic nitrates also produce systemic biochemical effects through altering neurohormonal status. These systemic effects may contribute significantly to the development of nitrate tolerance in therapeutic situations.

Animals↗
PubMed1992

A common enzyme may be responsible for the conversion of organic nitrates to nitric oxide in vascular microsomes.

We compared the nitric oxide (NO)-generating behavior of nitroglycerin (NTG), pentaerythritol trinitrate (PEtriN) and isosorbide dinitrate (ISDN), in the microsomal preparation of bovine coronary artery smooth muscle cells. The comparative NO generating activities among these nitrates were consistent with their relative reported vasodilating activities. Consistent with our previous observations with NTG, 400 microM bromosulfophthalein did not affect NO generation from PEtriN and ISDN in vascular microsomes while 400 microM 1-chloro-2,4-dinitrobenzene completely inhibited NO generation from these nitrates. Gel filtration chromatography with solubilized microsomes of bovine aortic smooth muscle cells showed the primary activity of NO generation from all three nitrates to be eluted at about 200 kD, consistent with that found with solubilized microsomes from the bovine coronary artery microsomes. These results suggest that organic nitrates may be converted to NO by one common enzyme in vascular microsomes.

Amino Acid Oxidoreductases↗
PubMed1992

Pharmacodynamic modeling of the in vitro vasodilating effects of organic mononitrates.

The in vitro dose-relaxation curves of four isomeric organic mononitrates: L-isoidide mononitrate (L-IIMN), isosorbide-2-mononitrate (IS-2-MN), isomannide mononitrate (IMMN), and isosorbide-5-mononitrate (IS-5-MN), were determined with rat aorta rings. These mononitrates relaxed vascular tissue in a concentration-dependent manner. Based upon the EC50 obtained from the Hill equation, the relative potency of L-IIMN: IS-2-MN: IMMN: IS-5-MN was 43.2:12.2:2.3:1. The Hill exponential coefficients were identical (value of about 1.5) in these four isomeric mononitrates, suggesting that they are likely to have a common mechanism of action. For all four isomers, relaxation was fairly immediate after addition of the tested compound into the tissue bath, with a 2- to 3-min-delay to reach steady-state effect. The rates of relaxation were then used to construct a pharmacodynamic model that described the time course of relaxation for these compounds. This theoretical analysis suggested that in vitro nitrate action is mediated by a catenary process, consistent with published biochemical evidence that suggests a series of reactions involving metabolic activation to nitric oxide, production of cyclic GMP, and myosin light-chain phosphorylation to produce relaxation. Via this pharmacodynamic model, the half-lives of nitric oxide and cGMP in the smooth muscle cells were estimated to be 15.2 and 23.1 sec, respectively, consistent with literature reports. Results from the present study indicated the potential use of in vitro pharmacodynamic modeling in confirming mechanism(s) of drug action obtained through biochemical or other methods.

Animals↗
PubMed1992

Structure activity relationship of organic nitrates: an exploratory hypothesis via molecular models.

An exploratory attempt to explain the structure/activity relationship of organic nitrates was initiated. In addition to the partition coefficient, a new empirical parameter, F(5.8A), was used in this correlation. This new parameter denotes the number of times that two oxygen atoms in the molecule can be arranged at a distance of 5.8A. Vasodilating potencies (EC50s) and partition coefficients of 11 organic nitrates were obtained from a literature report, while their corresponding F(5.8A) values were obtained from manual determination using two molecular models, viz: CPK and Drieding. Via linear regression comparisons, we showed that F(5.8A) was a better physicochemical parameter than the partition coefficient in explaining the differences in the observed pharmacologic activity. When the two parameters were combined, greater than 90% of the variability of the potency was accounted for. These results suggest that the pertinent protein (most likely the activating enzyme) for organic nitrate activity may contain at least two 'oxygen-philic' attachment sites which are separated by a distance of 5.8A.

Animals↗