Clinical pharmacokinetics of dapsone

The use of cimetidine to reduce dapsone-dependent methaemoglobinaemia in dermatitis herpetiformis patients

1. We have attempted to reduce dapsone-dependent methaemoglobinaemia formation in six dermatitis herpetiformis patients stabilised on dapsone by the co-administration of cimetidine. 2. In comparison with control, i.e. dapsone alone, methaemoglobinaemia due to dapsone fell by 27.3 +/- 6.7% and 26.6 +/- 5.6% the first and second weeks after commencement of cimetidine administration. The normally cyanotic appearance of the patient on the highest dose of dapsone (350 mg day-1), underwent marked improvement. 3. There was a significant increase in the trough plasma concentration of dapsone (2.8 +/- 0.8 x 10(-5)% dose ml-1) at day 21 in the presence of cimetidine compared with control (day 7, 1.9 +/- 0.6 x 10(-5)% dose ml-1, P less than 0.01). During the period of the study, dapsone-mediated control of the dermatitis herpetiformis in all six patients was unchanged. 4. Trough plasma concentrations of monoacetyl dapsone were significantly increased (P less than 0.05) at day 21 (1.9 +/- 1.0 x 10(-5)% dose ml-1) compared with day 7 (1.6 +/- 0.9 x 10(-5)% dose ml-1:control). 5. Over a 12 h period, 20.6 +/- 8.9% (day 0) of a dose of dapsone was detectable in urine as dapsone hydroxylamine. Significantly less dapsone hydroxylamine was recovered from urine at day 14 (15.0 +/- 8.4) in the presence of cimetidine, compared with day 0 (control: P less than 0.05). 6. The co-administration of cimetidine may be of value in increasing patient tolerance to dapsone, a widely used, effective, but comparatively toxic drug.

An investigation into the haematological toxicity of structural analogues of dapsone in-vivo and in-vitro

With microsomes prepared from a single human liver, 4,4'-diaminodiphenyl sulphone (DDS), 4-acetyl-4-aminodiphenyl sulphone (MADDS), 4-acetyl-4-aminodiphenyl thioether (MADDT) and 4,4'-diacetyldiphenyl thioether (DADDT) caused significantly greater methaemoglobin formation compared with control. In-vitro in the rat, the pattern of toxicity was slightly different:DADDT was not haemotoxic, whilst 3,4'-diaminodiphenyl sulphone (3,4'DDS) and 3,3'-diaminodiphenyl sulphone (3,3'DDS) as well as DDS, MADDS and MADDT were significantly greater than control. 4,4' Acetyl diphenyl sulphone (DADDS), 4,4' diaminodiphenyl thioether (DDT), 4,4'-diaminodiphenyl ether (DDE) and 4,4' diaminooctofluorodiphenyl sulphone (F8DDS) did not cause significant methaemoglobinaemia in either human or rat liver microsomes. DDS, MADDS, and MADDT were not significantly different in haemotoxicity generation in-vitro in the presence of human microsomes. In the rat in-vitro, DDS, MADDS, and 3,4'DDS did not differ significantly in red cell toxicity, and were the most potent methaemoglobin formers. The 3,3'DDS and MADDT derivatives were both significantly less toxic compared with DDS. None of the compounds tested caused haemoglobin oxidation in the absence of NADPH in-vitro. In the whole rat, DDS, MADDS and MADDT caused significantly higher levels of methaemoglobin compared with control. None of the remaining compounds caused methaemoglobin formation which was significantly greater than control. DDS and MADDS were the most potent methaemoglobin formers tested, in-vivo and in-vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of preincubation with cimetidine on the N-hydroxylation of dapsone by human liver microsomes

We have examined the ability of cimetidine to inhibit the oxidative metabolism and hence haemotoxicity of dapsone in vitro, using a two compartment system in which two Teflon chambers are separated by a semi-permeable membrane. Compartment A contained a drug metabolizing system (microsomes prepared from human or rat liver +/- NADPH), whilst compartment B contained human red cells. Preincubation (30 min) of human liver microsomes with cimetidine (0-1000 microM) and NADPH prior to the addition of dapsone (100 microM) and NADPH (1 mM) resulted in a concentration-dependent decrease in the concentrations of dapsone hydroxylamine (from 179 +/- 47 to 40 +/- 6 ng) in compartment B. This reduction of hydroxylamine metabolite was reflected in the concentration-dependent reduction in methaemoglobin measured (from 7.1 +/- 0.7 to 3.5 +/- 1.5%) in parallel experiments. Preincubation of microsomes with cimetidine in the absence of NADPH had no effect. The effect of cimetidine pretreatment on dapsone-dependent methaemoglobin was confirmed using microsomes prepared from a further three sources of human liver, as well as from rat liver.

Inhibition of dapsone-induced methaemoglobinaemia by cimetidine in the rat during chronic dapsone administration

Dapsone undergoes N-acetylation to monoacetyl dapsone as well as N-hydroxylation to a hydroxylamine which is responsible for the haemotoxicity (i.e. methaemoglobinaemia; Met Hb) of the drug. Since dapsone is always given chronically, we have investigated the ability of cimetidine to inhibit Met Hb formation caused by repeated dapsone administration. The drug was given (i.p.) to four groups (n = 6 per group) of male Wistar rats, 300-360 g. Group I received 10 mg kg-1 at 1, 24, 48 and 72 h. Group II received 10 mg kg-1 at 1, 8, 24, 32, 48, 56, 72 and 80 h. Groups III and IV received the drug as for groups I and II, respectively, as well as cimetidine (50 mg kg-1) 1 h before each dose of dapsone. Twice daily dapsone administration (Group II) resulted in a significantly greater (P less than 0.05) Met Hb AUC (757 +/- 135 vs 584 +/- 115% Met Hb h), dapsone AUC (140 +/- 17.5 vs 113 +/- 13.0 micrograms h mL-1) and monoacetyl dapsone AUC (48.2 +/- 18.3 vs 10.8 +/- 4.6 micrograms h mL-1) compared with a single daily dapsone dose (group I). The administration of cimetidine before the once daily dose of dapsone (group III) resulted in a significant (P less than 0.05) fall in Met Hb (302 +/- 179 vs 584 +/- 115% Met Hb h) and an increase in both the dapsone (151 +/- 22.2 vs 113 +/- 13.0 micrograms h mL-1) and monoacetyl dapsone AUC values (33.6 +/- 5.8 vs 10.8 +/- 4.0 micrograms h mL-1) compared with a single daily dose of dapsone (group I).(ABSTRACT TRUNCATED AT 250 WORDS)

Anti-Toxoplasma effects of dapsone alone and combined with pyrimethamine

The efficacy of dapsone alone or combined with pyrimethamine against Toxoplasma gondii was investigated experimentally. For in vitro studies, a sensitive immunoassay was used for assessment of Toxoplasma growth in tissue cultures; dapsone was found to have a significant inhibitory effect at a concentration of 0.5 micrograms/ml in the cultures, and the 50% inhibitory concentration was estimated to be 0.55 micrograms/ml. When pyrimethamine and dapsone were combined, an important synergistic effect which was associated with morphological alterations of the parasites was observed. In vivo studies were performed in a murine model of acute toxoplasmosis in which a tissue culture method was used to estimate the parasite burden in the blood, lungs, and brains of infected mice. Dapsone alone, which was administered at 100 mg/kg/day for 10 days from day 1 after infection, was unable to prevent parasite dissemination and only delayed the time to death of treated mice compared with the time of death of untreated controls. When dapsone and pyrimethamine (18.5 mg/kg/day) were administered in combination from day 4 after infection, parasites were cleared from blood and organs within 6 days, but relapses were observed 15 days after the cessation of therapy. When treatment was started at day 1 after infection, 100% of mice survived and relapses were not observed, suggesting a good efficacy of this combination for preventive therapy.

An investigation of the role of metabolism in dapsone-induced methaemoglobinaemia using a two compartment in vitro test system

1. We have utilized a two compartment system in which two teflon chambers are separated by a semi-permeable membrane in order to investigate the role of metabolism in dapsone-induced methaemoglobinaemia. Compartment A contained a drug metabolizing system (microsomes prepared from human liver +/- NADPH), whilst compartment B contained target cells (human red cells). 2. Incubation of dapsone (1-100 microM) with human liver microsomes (2 mg protein) and NADPH (1 mM) in compartment A (final volume 500 microliters) led to a concentration-dependent increase in the methaemoglobinaemia (15.4-18.9% at 100 microM) compared with control (2.3 +/- 0.4%) detected in the red cells within compartment B. In the absence of NADPH dapsone had no effect. 3. Of the putative dapsone metabolites investigated, only dapsone-hydroxylamine caused methaemoglobin formation in the absence of NADPH (40.6 +/- 6.3% with 100 microM). However, methaemoglobin was also detected when monoacetyl-dapsone, 4-amino-4'-nitro-diphenylsulphone and 4-aminoacetyl-4'-nitro-diphenylsulphone were incubated with human liver microsomes in the presence of NADPH. 4 Dapsone-dependent methaemoglobin formation was inhibited by addition of ketoconazole (1-1000 microM) to compartment A, with IC50 values of 285 and 806 microM for the two liver microsomal samples studied. In contrast, methaemoglobin formation was not inhibited by cimetidine or a number of drugs pharmacologically-related to dapsone. The presence of glutathione or ascorbate (500 microM) did not alter the level of methaemoglobin observed.

The use of cimetidine as a selective inhibitor of dapsone N-hydroxylation in man

1. The N-hydroxylation of dapsone is thought to be responsible for the methaemoglobinaemia and haemolysis associated with this drug. We wished to investigate the effect of concurrent administration of cimetidine (400 mg three times per day) on the disposition of a single dose (100 mg) of dapsone in seven healthy volunteers in order to inhibit selectively N-hydroxylation. 2. The AUC of dapsone (31.0 +/- 7.2 micrograms ml-1 h) was significantly increased (P less than 0.001) in the presence of cimetidine (43.3 +/- 8.8 micrograms ml-1 h). 3. Peak methaemoglobin levels observed after dapsone administration (2.5 +/- 0.6%) were significantly (P less than 0.05) reduced in the presence of cimetidine (0.98 +/- 0.35%). 4. The percentage of the dose excreted in urine as the glucuronide of dapsone hydroxylamine was significantly (P less than 0.05) reduced in the presence of cimetidine (34.2 +/- 9.3 vs 23.1 +/- 4.2%). 5. Concurrent cimetidine therapy might reduce some of the haematological side-effects of dapsone.

Gonadal influence on the metabolism and haematological toxicity of dapsone in the rat

Administration of dapsone (33 mg kg-1) to intact rats resulted in a marked elevation of methaemoglobin levels in male (435.0 +/- 105.2% met Hb h) compared with female rats (59.0 +/- 17.2% met Hb h). However, the clearance of dapsone was significantly faster in males compared with females. Female rats showed very low levels of methaemoglobin which were accompanied by significantly higher blood concentrations of parent drug. Clearance of dapsone in castrated animals was less than one-third of that of the intact sham-operated males (252.2 +/- 67.2 vs 81.4 +/- 33.0 mL h-1). Likewise, clearance of dapsone in ovarectomized rats was approximately half that of intact females. There were no significant differences in the disposition of dapsone between the ovarectomized (AUC 431.0 +/- 31.7 micrograms h mL-1; t1/2, 15.62 +/- 1.8 h) and castrated (AUC, 450.6 +/- 150.9 micrograms h mL-1; t1/2, 17.6 +/- 7.9 h) animals. However, methaemoglobin levels in castrated males, although less than a third of those of intact males, significantly exceeded those of ovarectomized animals. There was no significant difference between the four groups of animals with respect to red cell sensitivity to the methaemoglobin-forming capacity of the toxic metabolite of dapsone, the hydroxylamine. Metabolic conversion of dapsone to the hydroxylamine in the presence of NADPH was 7.6 +/- 1.5% for liver microsomes from intact males and was significantly greater (P less than 0.05) than the corresponding values for liver microsomes from castrated rats (5.3 +/- 0.59%). Conversion of dapsone to dapsone-NOH by liver microsomes from intact females and ovarectomized animals was below 1% in both cases.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple-dose pharmacokinetics and in vitro antimalarial activity of dapsone plus pyrimethamine (Maloprim) in man

1. The multiple-dose kinetics of dapsone (DDS), its major metabolite monoacetyldapsone (MADDS) and pyrimethamine (PYR) were studied in six healthy adult male volunteers following weekly administration of Maloprim (100 mg DDS plus 12.5 mg PYR). 2. After the last maintenance dose of Maloprim, the following kinetic parameters (mean values) were determined for DDS and PYR, respectively: maximum plasma concentration (Cmax) = 1,134 and 116 ng ml-1; elimination half-life (t1/2) = 23 and 105 h; plasma clearance (CL) = 37.6 and 15.9 ml h-1 kg-1 and apparent volume of distribution (Vss) = 1.20 and 2.29 l kg-1. The mean t1/2 of MADDS was 22 h. 3. The mean whole blood to plasma (B/P) and erythrocyte to plasma (E/P) concentration ratios for DDS were 1.04 and 1.09, respectively. MADDS had a B/P ratio of 0.69 and an E/P ratio of 0.33. The B/P and E/P ratios for PYR were 0.98 and 0.54, respectively. 4. The drug combination was assessed in vitro by measuring inhibition of re-invasion of two Plasmodium falciparum isolates grown in the presence of volunteers' sera. The chloroquine (CQ)- and PYR-sensitive FC-27 isolate was completely inhibited by the sera but the drug combination was ineffective against the CQ- and PYR-resistant K1 strain. The in vitro findings suggest that Maloprim may not be effective against strains of P. falciparum with a high level of resistance to pyrimethamine.

Inhibition of dapsone-induced methaemoglobinaemia in the rat isolated perfused liver

We have investigated the disposition of dapsone (DDS, 1 mg) in the rat isolated perfused liver in the absence and the presence of cimetidine (3 mg). After the addition of DDS alone to the liver there was a monoexponential decline of parent drug concentrations and rapid formation of DDS-NOH (within 10 min) which coincided with methaemoglobin formation (11.7 +/- 3.0%, mean +/- s.d.) which reached a maximum (22.6 +/- 9.2%) at 1 h. The appearance of monoacetyl DDS (MADDS) was not apparent until 30-45 min. Addition of cimetidine resulted in major changes in the pharmacokinetics of DDS and its metabolites. The AUC of DDS in the presence of cimetidine (1018.8 +/- 267.8 micrograms min mL-1) was almost three-fold higher than control (345.0 +/- 68.1 micrograms min mL-1, P less than 0.01). The half-life of DDS was also prolonged by cimetidine compared with control (117.0 +/- 48.2 min vs 51.2 +/- 22.9, P less than 0.05). The clearance of DDS (3.0 +/- 0.55 mL min-1) was greatly reduced in the presence of cimetidine (1.03 +/- 0.26 mL min-1 P less than 0.01). The AUC0-3h for DDS-NOH (28.3 +/- 21.2 micrograms min mL-1) was significantly reduced by cimetidine (8.1 +/- 3.40 micrograms min mL-1, P less than 0.01). In contrast, there was a marked increase in the AUC0-3h for MADDS (32.7 +/- 25.8 micrograms min mL-1) in the presence of cimetidine (166.0 +/- 26.5 micrograms min mL-1 P less than 0.01). The methaemoglobinaemia associated with DDS was reduced to below 5% by cimetidine. Hence, a shift in hepatic metabolism from bioactivation (N-hydroxylation) to detoxication (N-acetylation) caused by cimetidine, was associated with a fall in methaemoglobinaemia. These data suggest that the combination of DDS with a cytochrome P450 inhibitor might reduce the risk to benefit ratio of DDS.

Bioactivation of dapsone to a cytotoxic metabolite by human hepatic microsomal enzymes

1. Using human mononuclear leucocytes as target cells, we have investigated the bioactivation of dapsone (DDS) to a cytotoxic metabolite in the presence of microsomes from nine human livers. Values for NADPH dependent toxicity ranged from 8.8-27% (15.8 +/- 5.9%) and were similar to those for microsomes from control mice, 16-24% (19.0 +/- 4.8%). 2. Microsomes prepared from mice induced with either phenobarbitone or beta-naphthoflavone did not produce significantly more NADPH dependent toxicity than microsomes prepared from control mice. 3. Cytotoxicity was abolished not only by ascorbic acid, but also by sub-physiological concentrations of N-acetylcysteine and glutathione. 4. DDS was metabolised in vitro to a hydroxylamine (metabolic conversion 3.1 +/- 1.5%), which was oxidised further to a cytotoxic metabolite which also became irreversibly bound to protein.

Management of dapsone poisoning complicated by methaemoglobinaemia

The currently recommended dosage regimen for methylene blue (intermittent bolus dose) in the treatment of methaemoglobinaemia caused by dapsone is often inadequate. This is due to the long half-life of dapsone which provides a continuing oxidative stress that can cause a recurrence of clinically significant methaemoglobinaemia. Methylene blue infusion is effective, as demonstrated in an illustrative case report, and should be supported by repeated doses of activated charcoal to enhance dapsone elimination. The principles of treatment of methaemoglobinaemia due to dapsone can be applied to methaemoglobinaemia due to any agent producing prolonged oxidative stress.

Clinical pharmacokinetic considerations in the treatment of patients with leprosy

On the basis of the efficacy of the available agents, the World Health Organization has recommended only 4 drugs for combined chemotherapy of leprosy: rifampicin, dapsone, clofazimine and ethionamide/prothionamide. Thiacetazone and isoniazid are also used to a lesser extent by some physicians. Pyrazinamide may find a place in treating 'persister' bacilli. Dapsone is absorbed slowly after oral administration. Peak plasma drug concentration is reached at about 4 hours; absorption half-life is 1.1 hours; elimination half-life is about 30 hours. Oral availability is around 90%. Dapsone is approximately 70% protein-bound, while its monoacetylated metabolite is almost entirely bound. Dapsone crosses the placenta and is excreted into breast milk. It is metabolised via acetylation and N-hydroxylation, but acetylation polymorphism has no effect on dapsone handling by leprosy patients. Dapsone penetrates into sciatic nerves of experimental animals but its presence has not been demonstrated in Schwann cells. Oral doses of rifampicin are rapidly and completely absorbed. The bioavailability is greater when the drug is given before meals; peak concentrations occur at 1 to 2 hours. 80 to 90% of rifampicin is bound to plasma proteins, and the drug is found in saliva, cerebrospinal fluid and breast milk. Its main metabolite, desacetyl rifampicin, also exhibits antimycobacterial activity in tuberculosis. Rifampicin induces its own metabolism, as well as that of dapsone and steroids. Absorption of dapsone and rifampicin is reported to be reduced in leprosy patients. Clofazimine has been in use in leprosy treatment since 1960. In higher doses it exerts an anti-inflammatory action which is useful in treating leprosy patients in reaction. Oral absorption of the drug is slow and dose-dependent; faecal excretion also increases with dose. Single- and multiple-dose studies have shown a plasma half-life of around 10 days. Bioavailability of the drug is higher when given with food than when fasting; the peak plasma concentration occurs at 4 to 8 hours when the drug is administered with breakfast. After absorption, the drug is thought to circulate in protein-bound form, accounting for the fact that it is deposited in various tissues. Uneven distribution and prolonged retention in the tissues are special features of clofazimine metabolism. One unconjugated and 2 conjugated metabolites have been detected in urine, and the urinary excretion of both the parent compound and its metabolites is around 1% of the dose.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute and chronic drug-induced hepatitis

Adverse drug reactions may mimic almost any kind of liver disease. Acute hepatitis is often due to the formation of reactive metabolites in the liver. Despite several protective mechanisms (epoxide hydrolases, conjugation with glutathione), this formation may lead to predictable toxic hepatitis after hugh overdoses (e.g. paracetamol), or to idiosyncratic toxic hepatitis after therapeutic doses (e.g. isoniazid). Both genetic factors (e.g. constitutive levels of cytochrome P-450 isoenzymes, or defects in protective mechanisms) and acquired factors (e.g. malnutrition, or chronic intake of alcohol or other microsomal enzyme inducers) may explain the unique susceptibility of some patients. Formation of chemically reactive metabolites may also lead to allergic hepatitis, probably through immunization against plasma membrane protein epitopes modified by the covalent binding of the reactive metabolites. This may be the mechanism for acute hepatitis produced by many drugs (e.g. amineptine, erythromycin derivatives, halothane, imipramine, isaxonine, alpha-methyldopa, tienilic acid, etc.). Genetic defects in several protective mechanisms (e.g. epoxide hydrolase, acetylation) may explain the unique susceptibility of some patients, possibly by increasing exposure to allergenic, metabolite-altered plasma membrane protein epitopes. Like toxic idiosyncratic hepatitis, allergic hepatitis occurs in a few patients only. Unlike toxic hepatitis, allergic hepatitis is frequently associated with fever, rash or other hypersensitivity manifestations; it may be hepatocellular, mixed or cholestatic; it promptly recurs after inadvertent drug rechallenge. Lysosomal phospholipidosis occurs frequently with three antianginal drugs (diethylaminoethoxyhexestrol, amiodarone and perhexiline). These cationic, amphiphilic drugs may form phospholipid-drug complexes within lysosomes. Such complexes resist phospholipases and accumulate within enlarged lysosomes, forming myeloid figures. This phospholipidosis has little clinical importance. In a few patients, however, it is associated with alcoholic-like liver lesions leading to overt liver disease and, at times, cirrhosis. Subjects with a deficiency in a particular isoenzyme of cytochrome P-450 poorly metabolize perhexiline and are at higher risk of developing liver lesions. Prolonged, drug-induced liver-cell necrosis may also lead to subacute hepatitis, chronic hepatitis or even cirrhosis. This usually occurs when the drug administration is continued, either because the liver disease remains undetected or because its drug aetiology is overlooked. Several autoantibodies may be present.(ABSTRACT TRUNCATED AT 400 WORDS)

N-acetylation phenotyping with dapsone in a mainland Chinese population

1 The N-acetylation of dapsone (DDS) was studied in 108 unrelated Chinese subjects residing in the mainland of China. 2 The frequency of slow acetylators determined using the plasma monoacetyldapsone to DDS ratio (MADDS/DDS, slow acetylators less than 0.30 and rapid acetylators greater than 0.35) at 3 h after an oral dose of DDS (100 mg) was 13.0% (14 of the 108 subjects) with a 95% confidence interval of 7.9 to 20.6%. 3 The mean plasma concentration of MADDS was about three times lower in the slow than in the rapid acetylators and there was a highly significant correlation (rs = 0.886, P less than 0.001) between plasma MADDS concentration and acetylation ratio. 4 Urinary acetylation ratios (MADDS/DDS) and cumulative urinary excretion of MADDS were significantly (P less than 0.001) lower in slow acetylators compared with rapid acetylators. In addition, there was a significant relationship (rs = 0.510 to 0.718, P less than 0.001) between plasma and urinary acetylation ratios. However, the distribution of the urinary acetylation ratio was not bimodal. 5 The urinary acetylation ratio after an oral dose of DDS is not a discriminative index for determining acetylation phenotype.