Dapsone Metabolism Research Articles

DAPSONE-INDUCED METHEMOGLOBINEMIA TREATED WITH ORAL CIMETIDINE: A CASE REPORT

SESSION TITLE: Tuesday Medical Student/Resident Case Report Posters SESSION TYPE: Med Student/Res Case Rep Postr PRESENTED ON: 10/22/2019 01:00 PM - 02:00 PM INTRODUCTION: Dapsone is an antibiotic with anti-inflammatory properties commonly utilized in the treatment of various immunologic skin disorders. It is known to cause methemoglobinemia, a functional anemia characterized by an increased level of methemoglobin resulting in impaired oxygen release to tissues. We report a case of dapsone-induced methemoglobinemia successfully managed with oral cimetidine. CASE PRESENTATION: A 72-year-old male presented to the emergency department with the complaint of sudden onset of shortness of breath after undergoing a cardiac ablation three days prior to presentation. His past medical history included atrial fibrillation, cardiomyopathy, and an inflammatory skin condition known as granuloma annulare (GA). Of note, he was started on dapsone 100 mg daily for GA about a week prior to presentation. Pulse oximetry on admission demonstrated an oxygen saturation of 81 percent on room air despite the arterial blood gas (ABG) demonstrating a partial pressure of oxygen of 134 mm Hg. The patient’s methemoglobin level was noted to be elevated at 7.2 percent. Physical examination revealed crackles in bilateral lung flies. In addition, interstitial pulmonary edema was demonstrated on chest X-ray. The patient was admitted and treated for heart failure with furosemide. His home medications including dapsone were continued. On hospital day two, despite showing improvement in his pulmonary edema, the patient continued to require 12 liters of oxygen to maintain an adequate oxygen saturation. On hospital day 3, a repeat ABG revealed an increase in methemoglobin levels to 14.2 percent . Given these findings, dapsone-induced methemoglobinemia was suspected. Dapsone was discontinued and oral cimetidine 400 mg three times daily was initiated. About 22 hours after cimetidine was administered, methemoglobin level decreased to 5.5 percent. Methemoglobin continued to drop to 4.9 another 24 hours later. On hospital day six, the patient’s shortness of breath had completely resolved and his oxygen saturation improved to 95 percent on room air. No side effects to cimetidine were noted. The patient was discharged to home and prescribed cimetidine to be continued for one more week after discharge. DISCUSSION: Cimetidine, a histamine-2 receptor antagonist, can mitigate dapsone-induced methemoglobinemia by inhibiting the cytochrome P450 metabolism of dapsone to its toxic hydroxylamine metabolites, which are believed to be the cause of methemoglobinemia. Cimetidine can be given to prevent the further production of methemoglobin and thus further toxicity from dapsone. In this patient, a significant decrease in methemoglobin level and clinical improvement were observed within 24 hours of initiating cimetidine. CONCLUSIONS: This case provided evidence that oral cimetidine with discontinuation of dapsone may be an effective and well-tolerated option for the acute treatment of dapsone-induced methemoglobinemia. Reference #1: Ashurst JV, et al. Pathophysiologic mechanisms, diagnosis, and management of dapsone-induced methemoglobinemia. J Am Osteopath Assoc. 2010 Jan;110(1):16-20. Reference #2: Malfara WR. Effects of H(2)-receptor antagonists on dapsone-induced methaemoglobinaemia in rats. Pharmacol Res. 2002 Apr;45(4):269-73. Reference #3: Burke P, et al. Dapsone-induced methemoglobinemia: case of the blue lady. Can Fam Physician. 2013 Sep;59(9):958-61. DISCLOSURES: Speaker/Speaker's Bureau relationship with Allergan Please note: $5001 - $20000 Added 03/20/2019 by Frances Loftus, source=Web Response, value=Honoraria Speaker/Speaker's Bureau relationship with Boerhyinger ingelheim Please note: $1001 - $5000 Added 03/20/2019 by Frances Loftus, source=Web Response, value=Honoraria No relevant relationships by Natalie Millet, source=Web Response Speaker/Speaker's Bureau relationship with Melinta Pharmaceutical Please note: $1001 - $5000 Added 03/18/2019 by Joseph Reilly, source=Web Response, value=Honoraria

Hemolytic anemia in two patients with glioblastoma multiforme: A possible interaction between vorinostat and dapsone

Patients undergoing treatment for glioblastoma multiforme are routinely placed on prophylactic treatment for Pneumocystis jirovecii pneumonia because of significant therapy-induced lymphopenia. In patients with sulfa allergies, dapsone prophylaxis is often used due to its efficacy, long half-life, cost effectiveness, and general safety at low doses. However, dapsone may uncommonly induce a hemolytic anemia, particularly in patients deficient of glucose-6-phosphate dehydrogenase. This hemolysis is thought to be a result of oxidative stress on red blood cells induced by dapsone metabolites which produce reactive oxygen species that disrupt the red blood cell membrane and promote splenic sequestration. A single case report of dapsone-induced hemolytic anemia in a patient with glioblastoma multiforme has been reported. We present two patients with glioblastoma multiforme who developed severe hemolytic anemia shortly after initiating therapy with vorinostat, a pan-active histone deacetylase inhibitor, while on prophylactic dapsone. There are several potential mechanisms by which histone deacetylase inhibition may alter dapsone metabolism including changes in hepatic acetylation or N-glucuronidation leading to an increase in the bioavailability of dapsone's hematotoxic metabolites. In addition, vorinostat may lead to increased hemolysis through inhibition of heat shock protein-90, a chaperone protein that maintains the integrity of the red blood cell membrane cytoskeleton. The potential interaction between dapsone and vorinostat may have important clinical implications as more than 10 clinical trials evaluating drug combinations with vorinostat in patients with malignant glioma are either ongoing or planned in North America.

Radix Polygoni Multiflori Praeparata and Dioscorea Bulbifera Rhizome Decoctions Display Combined Effects Detected by a Three-Probe Drug Cocktail with Substrates of Rat Hepatic Cytochrome P450 Enzymes

Objectives: Radix Polygoni Multiflori Praeparata (RPMP) and Dioscorea Bulbifera Rhizomes (DBR) are used in Chinese herbal medicine and have been frequently reported for adverse reactions on liver. In this research, we aimed to evaluate in vivo effects of RPMP and DBR on rat cytochrome P450 enzymes (CYP1A2, CYP2E1 and CYP3A2) with their respective substrates as probes. Methods: Rats were orally administered RPMP, DBR and RPMP/DBR combination at 12, 10 and (12 + 10) g/kg, respectively, or saline as a control, once daily for 7 days. Thereafter, a cocktail containing 10 mg/kg caffeine, 20 mg/kg chlorzoxazone and 10 mg/kg dapsone was tail vein injected to rats. At defined time points, plasma drug concentrations were simultaneously evaluated by HPLC. Pharmacokinetic parameters simulated by DAS software were used to assess RPMP and/or DBR effects on cytochrome P450 enzymes activity. ANOVA and Dunnett’s test were used for data analysis. Results: Caffeine metabolism was enhanced in RPMP animals and reduced after pretreatment with DBR, but no effect was observed in RPMP/DBR combination group. Chlorzoxazone and dapsone metabolism was enhanced in both RPMP and DBR groups and consequently in combination group. The data suggested that RPMP independently induces rat CYP1A2, CYP2E1 and CYP3A2 activity, while DBR independently inhibits activity of rat CYP1A2 and induces that of CYP2E1 and CYP3A2. RPMP/DBR combination showed no significant benefit compared with the two drugs alone and even showed a neutralized effect in CYP1A2 activity. Conclusions: Caution is needed when RPMP and/or DBR are co-administered with drugs metabolized by human CYP1A2, CYP2E1 and CYP3A4.

Effects of ionophores on liver CYP1A and 3A in male broilers

The effects of ionophore antibiotics on the enzyme activity, protein and mRNA expression levels of cytochrome P450 (CYP) isoenzymes were investigated in liver from male Arbor Acres (AA) broiler chicks. Monensin, salinomycin and maduramycin at the dosage of 120, 60, and 5 mg/kg were administered in feed for 14 days. CYP1A and CYP3A activities were quantitated using cocktail probe drugs and a high performance liquid chromatographic (HPLC) assay at the 15th day; the protein expressions of CYP1A and CYP3A were detected by Western blot. CYP1A4, CYP1A5 and CYP3A37 mRNA levels were detected by real-time polymerase chain reaction (real-time PCR). Monensin, salinomycin and maduramycin had no effect on caffeine metabolism, protein expression and mRNA expression, but did induce dapsone metabolism, increasing CYP3A protein expression. However, there was no change in CYP3A37 mRNA expression as compared with the control group. It is suggested that ionophore antibiotics may have an induction effect on CYP3A expression and enzyme activity and that such effect might be related to the posttranscriptional regulation of its protein expression. Consideration of the enhanced metabolism of other drugs used simultaneously with ionophores is therefore recommended.

Cimetidine and dapsone‐mediated methaemoglobinaemia

I read with interest the case report from Choi et al. [1] relating to dapsone-mediated methaemoglobinaemia. The role of cimetidine in partially attenuating the haematological side-effects of dapsone does not appear to have been discussed in the anaesthesia literature. I wish to suggest the possible therapeutic benefit of peri-operative administration of cimetidine in patients receiving dapsone. The oxidative metabolism of dapsone leads to its toxicity. The majority of dapsone undergoes reversible acetylation in liver followed by elimination as N-hydroxylamine and to a lesser degree monoacetylated hydroxylamine. Dapsone N- hydroxylation is responsible for the haematological side-effects such as methaemoglobinaemia and haemolysis. Dapsone hydroxylamine depletes glutathione within glucose-6-phosphate dehydrogenase (G6PD)-deficient cells. The nitroso derivative then causes peroxidation reactions, leading to rapid haemolysis. Doses of 100 mg or less in normal healthy persons and 50 mg or less in G6PD-deficient individuals do not cause haemolysis [2]. A maximum dose of 150–300 mg a day, in divided doses, is recommended to minimise the risk of methaemoglobinaemia [3]. Cimetidine, an H2-recector antagonist, competes with dapsone for cytochrome P 450 enzymes. Thus cimetidine may be used as a selective inhibitor of N-hydroxylation and decrease the quantity of methaemoglobinaemia produced by dapsone [3, 4]. Cimetidine 400 mg three times daily has been shown to lead to a significant (> 25%) and sustained fall in methaemoglobin in patients on chronic dapsone therapy [5]. This dose appears to be well tolerated and not associated with adverse effects. The peri-operative use of cimetidine is likely to benefit patients by increasing the clinical safety margin, minimising the incidence and severity of methaemoglobinaemia and, possibly, avoiding the complications associated with treatment modalities such as methylene blue or ascorbic acid [6]. The mean elimination half-life of dapsone is about 20–30 h. Sulphones are retained in the circulation for long periods because of intestinal re-absorption from bile, requiring periodic interruption of treatment in dermatological diseases [2]. It is imperative to discontinue dapsone at least 4–7 days pre-operatively in patients listed for elective surgery. Furthermore, the G6PD level should be checked in all patients receiving dapsone therapy to identify patients at risk of peri-operative methaemoglobinaemia.

Effect of Conjugated Equine Estrogens on Oxidative Metabolism in Middle‐aged and Elderly Postmenopausal Women

The effects of conjugated equine estrogens (CEE) 0.625 mg daily on cytochrome P450 (CYP) were quantified in 12 middle-aged and 13 elderly postmenopausal women at baseline and 6 months later. CYP phenotype was characterized by caffeine (CYP1A2), chlorzoxazone (CYP2E1), dapsone (CYP, N-acetyltransferase 2), dextromethorphan (CYP2D6), and mephenytoin (CYP2C19) metabolism. CEE significantly decreased CYP1A2 (caffeine metabolic ratio: 0.57 +/- 0.20 before, 0.40 +/- 0.20 after, P = .001) and significantly increased CYP2D6 (dextromethorphan/dextrorphan ratio: 0.0116 +/- 0.0143 before, 0.0084 +/- 0.0135 after, P = .022) metabolism. CEE had no overall effect on CYP2C19, CYP2E1, CYP-mediated dapsone metabolism, and N-acetyltransferase 2. The dextromethorphan metabolic ratio decreased only in the seniors. The dapsone recovery ratio decreased in the middle-aged group and increased in the seniors. CEE significantly influenced CYP1A2, CYP2D6, and CYP-mediated dapsone oxidative metabolism but not CYP2C19, CYP2E1, or N-acetyltransferase 2 metabolism in postmenopausal women. Age influenced CYP2D6 metabolism and dapsone hydroxylation.

Dapsone acetylation in dermatitis herpetiformis.

The metabolism of dapsone was studied in ten patients with proven dermatitis herpetiformis. Seven of the patients, who were slow acetylators of sulphadimidine, also acetylated dapsone slowly. Three of the patients were rapid acetylators of both drugs. The control of symptoms did not seem to be related to the plasma concentrations of dapsone or of its metabolite monoacetyldapsone. Haemato-logical side effects, however, were related to plasma dapsone concentrations.

Effect of chronic disulfiram administration on the activities of CYP1A2, CYP2C19, CYP2D6, CYP2E1, and N-acetyltransferase in healthy human subjects.

Short-term disulfiram administration has been shown to selectively inhibit CYP2E1 activity but the effects of chronic disulfiram administration on the activities of drug metabolizing enzymes is unclear. The purpose of this study was to evaluate the effects of disulfiram given for 11 days on selected drug metabolizing enzyme activities. Seven healthy volunteers were given disulfiram 250 mg daily for 11 days. Activities of the drug metabolizing enzymes CYP1A2, CYP2C19, CYP2D6, CYP2E1 and N-acetyltransferase were determined using the probe drugs caffeine, mephenytoin, debrisoquine, chlorzoxazone, and dapsone, respectively. Chlorzoxazone was administered before disulfiram administration and after the second and eleventh doses of disulfiram, while the other probe drugs were given before disulfiram administration and after the eleventh disulfiram dose. Disulfiram administration markedly inhibited chlorzoxazone 6-hydroxylation by more than 95%, but did not affect metabolism of debrisoquine or mephenytoin. Caffeine N3-demethylation was decreased by 34% (P < 0.05). Monoacetyldapsone concentrations were markedly elevated by disulfiram administration resulting in a nearly 16-fold increase in the dapsone acetylation index, calculated as the plasma concentration ratio of monoacetyldapsone to dapsone. CYP-mediated dapsone N-hydroxylation was not significantly altered. These data suggest that disulfiram-mediated inhibition is predominantly selective for CYP2E1. The magnitude of CYP2E1 inhibition was similar after both acute and chronic disulfiram administration. The effects on caffeine N3-demethylation (CYP1A2) and dapsone metabolism suggest that chronic disulfiram administration may affect multiple drug metabolizing enzymes, which could potentially complicate the use of chronically administered disulfiram as a diagnostic inhibitor of CYP2E1.

The effect of pregnenolone 16alpha-carbonitrile on the pharmacokinetics and metabolism of dapsone in rats.

The purpose of this study was to evaluate the effect of pregnenolone 16 alpha-carbonitrile (PCN) on the interconversion pharmacokinetics and metabolism of dapsone. To determine microsomal CYP3A activity and protein, eight rats (4 PCN, 4 corn oil) received a 1 mg kg(-1) intravenous bolus dose of dapsone, followed by blood and urine sampling. The formation clearance of dapsone hydroxylamine (CLf DDS-NOH) was calculated from the obtained samples. Interconversion pharmacokinetics estimates were obtained after 10 rats (5 PCN, 5 control) received 1 mg kg(-1) dapsone or 1.17 mg kg(-1) monoacetyldapsone, with a 24-h wash-out. Results from the interconversion analysis demonstrated that PCN significantly increased systemic clearance (CLs) of dapsone, but not its interconversion. The in-vivo/in-vitro correlation study demonstrated that PCN significantly increased CLs of dapsone (8.55 to 16.39mLmin(-1); P<0.01) and CLf DDS-NOH (0.13 to 0.18mLmin(-1); P<0.01). PCN treatment produced a 69% increase in CYP3A protein, and increased 6beta- and 2beta-hydroxytestosterone formation rates. Significant correlations were found between CLf DDS-NOH and either 6beta- (r2 = 0.925), 2beta-hydroxytestosterone (r2 = 0.92), or CYP3A1/2 protein (r2= 0.60). We conclude that PCN treatment produces significant increases in CLs (dapsone) and CLf (DDS-NOH) in rats. These changes were not due to changes in the reversible metabolism of dapsone. These results suggest that the formation clearance of dapsone hydroxylamine reflects alterations in CYP3A activity, despite the fact that it accounted for a small part of the systemic clearance of dapsone.

Development of dapsone toxicity in patients with inflammatory dermatoses: activity of acetylation and hydroxylation of dapsone as risk factors.

Alternative independent routes of dapsone metabolism include N-hydroxylation to the hydroxylamine, a potentially toxic metabolite, by cytochrome P450 enzymes and acetylation to a nontoxic metabolite by N-acetyltransferase. Potentially, therefore, the relative extents of these two routes in an individual could determine the occurrence of adverse reaction with dapsone therapy. Phenotypic activity of these two routes of metabolism was assessed in 18 patients receiving longterm dapsone therapy for inflammatory dermatoses and was related to the development of dapsone toxicity. N-Hydroxylation was assessed by the dapsone recovery ratio, a ratio of dapsone hydroxylamine to the sum of hydroxylamine and dapsone in 8-hour urine, whereas N-acetylation was assessed by the acetylation ratio, a ratio of monoacetyldapsone to dapsone in 8-hour plasma sample after an oral dose of dapsone. There was wide intersubject variation in both the acetylation ratio and the dapsone recovery ratio, but both phenotypic measures remained stable within individuals. The dapsone recovery ratio showed a tendency toward being lower in fast than in slow acetylators, but this was not statistically significant. There was an inverse relationship between acetylation and hydroxylation (r = -0.69; P < .005) at steady state that was not apparent after the first dose. Neurotoxicity developed in two subjects and hemolytic anemia developed in two subjects. Plasma levels of dapsone in these four subjects were similar to those in subjects who showed no toxicity. All four were slow acetylators and three were rapid hydroxylators, consistent with the toxic nature of dapsone hydroxylamine. These observations are consistent with what is known about the toxicity profile of dapsone metabolites and suggest that assessing N-acetylation and N-hydroxylation capacities can help to identify subjects at increased risk of a toxic response. This approach of assessing the phenotypic measures of drug-metabolizing activity to predict adverse reaction may also apply to other drugs with metabolic-based adverse effects.

Evaluation of atypical cytochrome P450 kinetics with two-substrate models: evidence that multiple substrates can simultaneously bind to cytochrome P450 active sites.

Some cytochrome P450 catalyzed reactions show atypical kinetics, and these kinetic processes can be grouped into five categories: activation, autoactivation, partial inhibition, substrate inhibition, and biphasic saturation curves. A two-site model in which the enzyme can bind two substrate molecules simultaneously is presented which can be used to describe all of these observed kinetic properties. Sigmoidal kinetic characteristics were observed for carbamazepine metabolism by CYP3A4 and naphthalene metabolism by CYPs 2B6, 2C8, 2C9, and 3A5 as well as dapsone metabolism by CYP2C9. Naphthalene metabolism by CYP3A4 and naproxen metabolism by CYP2C9 demonstrated nonhyperbolic enzyme kinetics suggestive of a low Km, low Vmax component for the first substrate molecule and a high Km, high Vmax component for the second substrate molecule. 7, 8-Benzoflavone activation of phenanthrene metabolism by CYP3A4 and dapsone activation of flurbiprofen and naproxen metabolism by CYP2C9 were also observed. Furthermore, partial inhibition of 7, 8-benzoflavone metabolism by phenanthrene was observed. These results demonstrate that various P450 isoforms may exhibit atypical enzyme kinetics depending on the substrate(s) employed and that these results may be explained by a model which includes simultaneous binding of two substrate molecules in the active site.

Studies on the toxicity of analogues of dapsone in-vitro using rat, human and heterologously expressed metabolizing systems.

Three metabolizing systems (rat, heterologously expressed CYP3A4 and human liver) were used to evaluate 12 analogues of dapsone (4,4'diaminodiphenylsulphone) in-vitro. Methaemoglobin formation in a two-compartment and cytotoxicity in a single-compartment model were studied using human erythrocytes and neutrophils, respectively, as target cells. In the two-compartment system using rat microsomes as a generating system and methaemoglobin as an endpoint, the least potent methaemoglobin formers tested were the 2-methyl-4-propylamino (AXDD14), 2-hydroxy-4-4'amino (ABDD5) derivatives and a sulphone/trimethoprim derivative (K-130). Dapsone itself, a 2-methoxy-4-ethylamino (W10) and a 2-hydroxyl-4-ethylamino compound (ABDD39) were the most toxic. In the single-compartment cytotoxicity test using rat microsomes, AXDD14 was again among the least toxic, as was a 2-methyl 4-cyclopentyl derivative (AXDD17) and surprisingly ABDD39. The most cytotoxic compounds again included dapsone itself as well as two 2-trifluoromethyl derivatives. The only significant methaemoglobin formation and cytotoxicity shown with the heterologously expressed human CYP 3A4 was with AXDD14, which was extensively activated. Interestingly, metabolism of dapsone was low using the expressed CYP 3A4. In the two-compartment system using human liver microsomes, AXDD14, K-130 and ABDD5 were oxidized to a significantly lesser extent compared with dapsone and these preliminary findings indicate that future development of these compounds may be worthwhile.

Alteration of dapsone metabolism by rifabutin in HIV(+) subjects.

Alteration of dapsone metabolism by rifabutin in HIV(+) subjects.

Metabolism of dapsone to its hydroxylamine by CYP2E1 in vitro and in vivo*

Dapsone toxicity is putatively initiated by formation of a hydroxylamine metabolite by cytochromes P450. In human liver microsomes, the kinetics of P450-catalyzed N-oxidation of dapsone were biphasic, with the Michaelis-Menten constants of 0.14 +/- 0.05 and 0.004 +/- 0.003 mmol/L and the respective maximum velocities of 1.3 +/- 0.1 and 0.13 +/- 0.04 nmol/mg protein/min (mean +/- SEM). Troleandomycin (40 mumol/L) inhibited hydroxylamine formation at 100 mumol/L dapsone by 50%; diethyldithiocarbamate (150 mumol/L) and tolbutamide (400 mumol/L) inhibited at 5 mumol/L dapsone by 50% and 20%, respectively, suggesting that the low-affinity isozyme is CYP3A4 and the high-affinity isozymes are 2E1 and 2C. Disulfiram, 500 mg, 18 hours before a 100 mg oral dose of dapsone in healthy volunteers, diminished area under the hydroxylamine plasma concentration-time curve by 65%, apparent formation clearance of the hydroxylamine by 71%, and clearance of dapsone by 26%. Disulfiram produced a 78% lower concentration of methemoglobin 8 hours after dapsone.

The use of a three compartment in vitro model to investigate the role of hepatic drug metabolism in drug-induced blood dyscrasias.

1. N-hydroxylation is thought to be an essential step in the haemotoxicity of dapsone (DDS). To investigate both metabolism-dependent and cell-selective drug toxicity in vitro we have developed a three-compartment system in which an hepatic drug metabolizing system is contained within a central compartment separated by semipermeable membranes from compartments containing mononuclear leucocytes (MNL) and red blood cells (RBC). 2. Metabolism of dapsone (100 microM) by rat liver microsomes resulted in toxicity to RBC cells (47.3 +/- 2.1% methaemoglobin), but there was no significant toxicity toward MNL (3.7 +/- 1.3% cell death) compared with control values (1.6 +/- 0.9%). However, when RBC were replaced with buffer in the third compartment there was significantly greater (P < 0.001) white cell toxicity (17.6 +/- 0.6% cell death), demonstrating the protection of MNL by RBC. Metabolism of dapsone by human liver microsomes again resulted in RBC toxicity (12.5 +/- 3.3% methaemoglobin) but no significant MNL toxicity (2.9 +/- 0.8% cell death). Replacement of RBC resulted in a significant (P < 0.001) increase in MNL toxicity (6.5 +/- 0.7% cell death). Addition of synthetic dapsone hydroxylamine (30 microM) in the absence of a metabolizing system and with no RBC in the third compartment resulted in significant (P < 0.001) toxicity toward MNL (43.36 +/- 5.82% cell death) compared with control (1.8 +/- 1.1%). The presence of RBC in the third compartment resulted in a significant (P < 0.001) decrease in MNL toxicity (17.6 +/- 2.2% cell death), with 40.1 +/- 3.7% methaemoglobin in the RBC.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetic optimisation in the treatment of Pneumocystis carinii pneumonia.

Several drugs and drug combinations are currently used in the treatment of patients with Pneumocystis carinii pneumonia (PCP)--pentamidine and cotrimoxazole (trimethoprim plus sulphamethoxazole), which are indicated for this usage, dapsone/trimethoprim and clindamycin/primaquine, which are not licensed for PCP, and trimetrexate/calcium folinate (leucovorin), eflornithine and BW-566C (566 C80) as investigational drugs. For most of these agents, recommendations regarding the use of pharmacokinetic parameters to establish individualised therapy cannot be made. The pharmacokinetics of antipneumocystis drugs are not well documented and clinical trials evaluating the relationship between the individual plasma pharmacokinetic profiles and responses to treatment are sparse. In clinical trials, the reduction of the daily dose of pentamidine to 3 or 2 mg/kg/day and of cotrimoxazole to 15 mg/kg of the trimethoprim component resulted in a substantial reduction of frequency and severity of adverse drug effects without diminishing efficacy. For pentamidine, a long half-life of > or = 4 days implies the need for a loading dose. Plasma concentrations of the parent drug at steady-state varied between 30 and 100 micrograms/L. The elimination pharmacokinetics are characterised by several elimination slopes indicating the existence of a deep peripheral compartment. Due to its very low renal clearance, dosage adjustments are not necessary in patients with renal impairment. The pharmacokinetics of cotrimoxazole follow first-order kinetics in PCP and the particular parameters are similar to those reported in the treatment of bacterial infection. Steady-state plasma concentrations of both trimethoprim and sulphamethoxazole are attained within 2 to 3 days, but the range of 'therapeutic' plasma concentrations must be newly defined, since elevated trimethoprim concentrations could not be correlated with the frequency and severity of adverse drug reactions. The concentrations of sulphamethoxazole may be at least as important as those of trimethoprim in defining a toxic range. With dapsone/trimethoprim, clindamycin/primaquine and BW-566C (566 C 80) good clinical response rates were found in groups of patients with mild to moderate PCP. Comparative trials with standard drugs are still ongoing. Therapeutic to toxic concentration ratios have not been established in patients with PCP. Pharmacokinetic data pertaining to patients with PCP are either nonexistent or incomplete, or are complicated by a drug interaction between dapsone and trimethoprim suggesting an inhibition of metabolism of dapsone. Eflornithine and trimetrexate/calcium folinate have been used under specific research protocols, showing partial success as salvage agents for desperately ill patients with AIDS. Regarding all antipneumocystis drugs, additional clinical and pharmacokinetic data are needed to optimise and more fully individualise the treatment regimens for this severe infection.

Acetylation polymorphism and leprosy

The sulfones are the drug of choice in the treatment of leprosy, with dapsone as the clear favorite. The major route for dapsone metabolism leading to its inactivation and excretion is via acetylation by hepatic N-acetyl transferase (NAT), as is the case with isoniazid (INH) and sulfamethazine (SMZ). The enzyme is known to exhibit genetic polymorphism. The object of the present study is mainly to determine the incidence of acetylator phenotype in a population of leprosy patients with a view to evaluating the degree of association, if any, between phenotype and the disease. Obviously a knowledge of the incidence of the phenotypes may provide a valuable contribution to the institution of more rational and successful therapy. In the normal or control subjects, as well as in the leprosy patients, the frequency distribution histograms of the percentage acetylsulfamethazine in urine and serum samples are bimodal, and this indicates the existence of a genetic polymorphism. Based on the bimodality, individuals were classified as either "rapid" or "slow" acetylators, and the incidence of the slow acetylator phenotype of about 51% was observed in the leprosy population. This gives a relatively high incidence of the allele controlling the slow acetylator (q = 0.73). Although there is evidence that the mean percentage of SMZ acetylated in leprosy patients of the slow acetylator phenotype is significantly higher than that observed for the same phenotype in the controls (t = 4.86, P less than 0.02), statistical analyses show that there is no association between the slow acetylator phenotype and the disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Dapsone Metabolism in Patients with Dapsone-Resistant Leprosy

Acetylation of dapsone (DDS) and sulfamethazine (SMZ), and plasma clearance of DDS were studied in Malaysian Chinese with lepromatous leprosy including 40 DDS-resistant and 44 non-resistant patients. Neither a patient's acetylation characteristics (DDS or SMZ), nor his plasma clearance rate, appeared to have predisposed him to the development of DDS resistance. A potentially important drug interaction between rifampin and DDS was discovered. After ingestion of rifampin for a minimum of 2 weeks, the plasma clearance of DDS was increased and the relative amount of the acetylated DDS was decreased. The implications of these results for the treatment of lepromatous leprosy are discussed.

N-hydroxylation of 4,4'-diaminodiphenylsulphone (Dapsone) by liver microsomes, and in dogs and humans.

1. During the incubation of Dapsone1 with rabbit liver microsomes, NADPH and bovine erythrocytes, rapid haemoglobin oxidation was observed. The velocity increased 2–3 times with liver microsomes of rabbits pretreated with phenobarbital. 2. Liver microsomes of rabbits catalyzed the N-hydroxylation of DDS in the presence of O2 and NADPH. The oxidation was dependent upon microsomal protein, DDS concentration, NADPH concentration and pH. The velocity of N-hydroxylation in incubates with microsomes from rabbits pretreated with phenobarbital was 2–3 times greater than the velocity with microsomes from control animals. Carbon monoxide and metyrapone inhibited the microsomal N-hydroxylation of DDS. The reaction must be included in the cytochrome P-450 dependent N-hydroxylations of primary arylamines. 3. In dogs, very low amounts of free DDS-NOH were found in the urine. 7–10% of an oral dose of 50 mg/kg DDS was excreted in the urine in the form of conjugated DDS-NOH liberated by acid hydrolysis (1 N HCl at 20°C). 4. Human volunteers receiving 200 mg DDS in capsules excreted 0.9–3.4% of the dose as free DDS-NOH and 6–20% as conjugated, acid labile DDS-NOH within 24h. After 72 h 5–7% of the dose was excreted as free DDS-NOH and 25–33% as conjugated, acid labile DDS-NOH. 80–90% of the DDS-NOH conjugates were liberated by treatment with 1 N HCl at 20°C. The total amount of conjugates in the urine was split by glusulase treatment under anaerobic condition. N-Hydroxy metabolites of DDS in the urine can reach 50% of the dose. Dapsone metabolism in humans is the first example in which N-hydroxylation is the principal metabolic pathway.