Furthermore, the influence of several co‐variates such as creatinine clearance, liver function tests, body weight and body surface area, on pharmacokinetics of heroin and its metabolites was studied in this randomised, double blind study. The correlation between plasma levels and subjective appreciation of heroin use or craving could therefore be studied unbiased. Furthermore, reaction time tests, blood pressure, skin temperature and heart rate were assessed.

Metabolism of heroin (diacetylmorphine) and its major metabolites. Heroin is hydrolysed to 6‐monoacetylmorphine and morphine. Glucuronides are conjugated to the 3′ or 6′ position of the phenantrene ring.

Heroin is a semi‐synthetic morphine ester, with two acetyl groups coupled to the 3‐ and 6‐hydroxyl groups of morphine ( fig. 1 ). Esters like heroin are thought to pass the blood‐brain barrier more rapidly than its precursor product morphine, and would therefore account for a larger pharmacodynamic potency compared to morphine ( Oldendorf et al. 1972 ). After absorption, heroin is rapidly hydrolysed to 6‐monoacetylmorphine and morphine by serum and liver esterases. Furthermore, heroin metabolite morphine is conjugated to glucuronides ( fig. 1 ). Heroin is considered as a prodrug that mainly acts by its agonistic metabolites 6‐monoacetylmorphine (6‐MAM), morphine (MOR) and morphine‐6‐glucuronide (M6G)( Inturrisi et al. 1983 ; Selley et al. 2001 ). The major metabolite, morphine‐3‐glucuronide (M3G) has virtually no affinity for μ‐opioid receptors, and therefore displays no agonistic activity. M3G accumulation is thought to be related to neurotoxic effect of long‐term morphine use ( Smith 2000 ). Since metabolites of heroin plays an important role in efficacy and toxicity of heroin, not only the pharmacokinetic parameters of heroin, but also those from the major metabolites were investigated in this pharmacokinetic‐pharmacodynamic study. To study linearity of heroin kinetics, steady‐state heroin doses were varied with a dose increment of plus 50%, and a dose reduction of minus 33%.

Although heroin administration by “chasing the dragon” appears to be a feasible treatment like intravenous heroin administration, not much is known about the pharmacokinetic profile of this specific administration method. In two studies, estimation of bioavailability of heroin inhalation was based on urine data ( Mo & Way 1966 ; Hendriks et al. 2001 ). Jenkins et al. (1994) described the pharmacokinetic features of a single draught of heroin vapours in two addicts, based on plasma data ( Jenkins et al. 1994 ). To our knowledge, no pharmacokinetic data of repeated heroin inhalation by chasers based on plasma concentration time curves in human beings are available. Therefore, a pharmacokinetic‐pharmacodynamic study was performed on a group of heroin inhalers, who were treated in the Dutch heroin‐assisted treatment programme. Furthermore, such data were also collected from intravenous users in a parallel study with similar design.

Beside intravenous use, illicit heroin is also used by a smoking technique called “chasing the dragon”. In chasing the dragon, heroin base is heated on tin foil above a cigarette lighter, controlled by the patient. The heroin vapours generated by heating are inhaled into the lungs, using a straw in the mouth. The name of this smoking technique refers to the way opium was smoked by the Chinese in the 1950's ( Strang et al. 1997 ). Epidemiological studies in heroin addicts have consistently indicated that “chasing the dragon” is nowadays the predominant route with heroin self‐administration in The Netherlands ( Hartgers et al. 1991 ). Therefore, in the Dutch heroin‐assisted treatment programme, heroin hydrochloride for injection and heroin base for inhalation by “chasing the dragon” technique were used in two parallel studies. In this article, heroin inhalation refers to smoking heroin by “chasing the dragon”.

Diacetylmorphine, or pharmaceutically prepared heroin, is prescribed in several European countries as maintenance treatment for patients with severe heroin addiction, who failed in conventional treatments. In several clinical trials, high doses of heroin have been administered to opioid‐addicted patients, under stringent surveillance in special out‐patient clinics. Heroin‐assisted treatment programs in the UK, Switzerland and The Netherlands have shown good feasibility ( Perneger et al. 1998 ; van den Brink et al. 2003 ). The Swiss and Dutch studies provided evidence that treatment with heroin on medical prescription significantly improved mental, social and physical condition of heroin‐addicted patients. Compliance and responding rates were high in these studies. Heroin‐assisted treatment is currently considered in other countries e.g. Spain, Germany, Belgium and Canada ( Fischer et al. 2002 ).

Materials and Methods

Patients. Male patients, who were on combined heroin with methadone treatment for a period of at least 12 months, and who responded well to this treatment according to the predefined criteria in the protocol of the Dutch Heroin on Medical Prescription Research Project, participated in the study (for criteria, see van den Brink et al.(2003)).

For this pharmacokinetic‐pharmacodynamic study, the participants had to have reasonable adequate haematological, renal and hepatic functions before entering the study (haemoglobin >8.0 mmol/l, serum creatinine concentration <125 μmol/l, serum gamma glucuronyl transferase (GGT) and liver amino transferases ASAT and ALAT less than twice the upper limit of normal). Patients who were on treatment with HIV medication, cimetidine or MAO‐inhibitors were excluded. All other co‐medication besides the prescribed opioids was allowed, if this medication was intended for chronic use, and no dose changes had been made for a period of at least one month before entering the study.

Body mass index was calculated from each participant by dividing weight (in kilograms) by square length (in meters). Body surface area was calculated following the DuBois‐DuBois formula. Renal functioning of each participant was calculated from serum creatinine following Cockcroft‐Gault equation.

Participation on this study was completely voluntary, and written informed consent before entering the study was obtained from all patients. Protocol and consent form were approved before the start of the study by the Committee of Medical Ethics of the Slotervaart Hospital in Amsterdam. The study was performed according to Good Clinical Practice regulations.

Design and setting. Participants were hospitalised in a research clinic, during the 4 day trial (Kendle Clinical Pharmacology Unit, Utrecht, the Netherlands). They were all on steady‐state treatment with heroin and methadone for at least 12 months, and received heroin twice daily and methadone once daily. The maximal sustained single heroin dose for entering this study was 300 mg. No limitations in the methadone dose were set. Methadone and heroin dosage regimens had to be constant at least four weeks before entering this study. In this study, regular methadone dosages were given once daily, 2 hr after the heroin morning administration. The dose interval between the heroin morning and evening dose was 8 hr. Pharmacokinetic and pharmacodynamic assessments were performed from 1 hr before to 8 hr after each heroin morning dose.

On the first day of the study, regular steady‐state heroin doses for the particular patient were given. On the following three days, heroin morning dose levels were varied double blind from 67–100–150%, in a random order. The heroin evening dose remained unchanged during the study.

Intravenous users were asked to administer heroin by autoinjection, in one fluent movement as a bolus. Heroin inhalers got a period of maximal 30 min. to inhale the heroin dosage. This period of 30 min. is in accordance with the maximal sustained inhalation time in the outpatient clinics. For blinding reasons, heroin base was presented in a scattered way on a piece of tin foil of 10×20 cm. Furthermore, the heroin inhalation dose was offered in two unequal portions of respectively 40% and 60% of the total dose. A 4 min. pause was scheduled when the first portion was completely inhaled. In this pausing period, a blood sample was drawn and some pharmacodynamic assessments took place.

The use of alcohol, cannabis, cocaine, and opiates besides trial medication was not allowed during the trial period. Luggage was checked for these items. The study took place in a closed ward, and visitors were not allowed. Alcohol breath tests were taken before each heroin morning dose during the study. All plasma samples were controlled for cocaine use.

Plasma sampling and bioanalysis. Blood samples were drawn from an intravenous cannulla, placed in the underarm. In intravenous users, the cannulla was placed in the arm opposite to the injection site. Blood samples were taken 10 min. before start of heroin administration and at 2, 5, 10, 15, 30, 45, 60, 115, 180, 240 and 480 min. after heroin administration. For inhalers, an additional sample was collected 2 min. after 40% of the heroin dose was used.

Blood was sampled in 6 ml glass tubes, containing 15 mg sodium fluoride and 12 mg potassium oxalate. The tubes were put in ice water, immediately after sampling. Within 15 min. after sampling, the blood samples were centrifuged at 2000×g for a period of 5 min. The plasma fraction was collected and snap‐frozen. Plasma samples were kept at −30 °, and defrosted in ice water before bioanalysis. Methadone, heroin and its metabolites 6‐MAM, MOR, M3G and M6G were quantified simultaneously, by LC‐MS‐MS. The lowest limit of quantification (LLQ) was 5 ng/ml, for all quantified analytes. The accuracy, and precision of the analysis method met the current standards of the Food and Drug Administration Guidelines. For further details regarding the bioanalytical method, see Rook et al. (2005).

In the same analytical run, cocaine and its metabolites norcocaine and benzoylecgonine were qualitatively detected in plasma to verify whether the subject had used cocaine before or during the study.

Medication. All heroin formulations used in this study were similar to the formulations that are normally used in outpatients' settings. Pharmaceutically prepared heroin base for inhalation, and heroin hydrochloride for injection, were produced under Good Manufacturing Practice conditions. For heroin inhalers, caffeine was added to the heroin base formulation in a 1:3 ratio. Caffeine is known to enhance the volatilisation of heroin, by lowering the melting point of heroin (Huizer 1987). Heroin hydrochloride for injection was dissolved in 3 ml Water for Injection, immediate before administration, in a laminar airflow hood under sterile conditions. For blinding reasons, all dosages were dissolved in the same volume of Water for Injection. Racemic methadone syrup came from the hospital pharmacy. Other co‐medication besides heroin and methadone came from community pharmacies.

Pharmacokinetic analysis. Pharmacokinetic parameters for all compounds were calculated by non‐compartmental analysis, using WinNonlin software (Standard Edition Version 3.0, 1999, Pharsight, Mountain View, CA, USA). The area under the curve of the observed concentrations (AUC) was determined by the log‐linear interpolated trapezoid rule, with extrapolation to infinity by the elimination constant k e (slope of terminal part of the concentration‐time‐curve on semi‐log scale). When the plasma concentrations of the analytes were above the LLQ at the null measurement (C 0 ), the AUC was corrected by subtracting C 0 /k e .

The bioavailability (F) of heroin by the inhalation route was estimated using the following equation:

Clearance (Cl) and distribution volume (Vd) were calculated for intravenous heroin data, and Cl/F and Vd/F were calculated for inhaled heroin. Since it is known that virtually all heroin is extensively metabolised into 6‐MAM, without loss of heroin into urine, the virtual dose of 6‐MAM would be practically similar to the actual heroin dose (Yeh et al. 1976). Cl/(F) and Vd/(F) were therefore also calculated for the metabolite 6‐MAM.

AUC 0‐inf and elimination half‐life (T 1/2 were calculated for heroin and all its measured metabolites. The maximal plasma concentrations (C max ) of heroin and its metabolites and time point at which the C max was measured (T max ) were derived directly from plasma‐concentration‐time curves.

Pharmacodynamic assessments. Simple reaction time(SRT) was measured by a computerised neuropsychological test battery (FePsy version 6.5, Stichting Epilepsie Instellingen Nederland, Heemstede, The Netherlands). Reaction times to single visual stimuli were measured. In one session, 30 stimuli were presented, both for the dominant and the non‐dominant hand. The inter‐stimuli intervals varied at random from 2.5 to 4 sec. Stimulus exposure continued until a response, by pushing a button, was given by the subject. The reaction times are expressed in millisec. (msec.). Reaction time assessments were taken 20 min. before heroin administration and 20, 100, 150 and 260 min. after heroin administration.

Blood pressure, heart rate and skin temperature. Systolic and diastolic blood pressure, and heart rate were measured non‐invasively and simultaneously 60 min. before and 13, 28, 70, 110, 145 and 250 min. after heroin administration. Heart rate was derived from lead 2 of the ECG. Heart rate, and blood pressure, were measured after a 3 min. resting period in supine position. To assess the effects of heroin use under orthostatic conditions, blood pressure and heart rate were also measured after standing erect for 1 min.; from the supine position. Orthostatic tension was calculated as the difference between systolic pressure, after orthostatic stress, minus systolic pressure, measured in supine position.

Skin temperature was measured simultaneously with cardiovascular functions, by a sensor tapered on the hand. All assessments were taken with Datex‐Ohmeda S/5 light monitor equipment (Datex‐Ohmeda Inc, Tewksbury, MA, USA).

Subjective effects. Ratings of subjective drug effects were measured by means of visual analogue scales (VAS). The rates of appreciation of the effects of the heroin intake were measured on a scale varying from −10 cm (“very bad effect”) to +10 cm (“extremely good effect”). The appreciation VAS scales were collected in the pre‐scheduled pause during the inhalation sessions, and 6, 35, 90 and 260 min. respectively after completely finishing heroin inhalation or injection.

Craving for heroin use was measured on a VAS from 0 (none craving at all) to 20 cm (feelings of extreme craving). Craving was measured 30 min. before heroin morning administration, and 6 and 50 min. after heroin morning administration.

The Physical Symptom Questionnaire (PSQ) contained subscales for withdrawal symptoms (7 items), positive drug effects (3 items) and signs of overdose (8 items)(Hendriks et al. 2001). Withdrawal symptoms that were listed were “feeling cold”, “running nose”, “muscular tension”, “muscular aching”, “gooseflesh”, “yawning”, and “restlessness”. The following positive drug effects, “feeling high”, “pleasant feeling in stomach”, “itchy nose”, and the following overdose symptoms, “giddiness”, “light‐headedness”, “sleeping limbs”, “tickling”, “sweating”, “nausea”, “squeezed throat”, and “palpitations”, were listed. The scores for each item varied from zero (no sign at all) to three (extreme). The maximal scores for the subscales were respectively 21, 9 and 24. PSQ was measured at baseline and at 6, 35, 90 and 260 min. after heroin administration.

Inquiries after adverse events took place at base level and respectively 15 min., 2 and 12 hr after heroin intake. Serious adverse events were defined as any symptom that compelled immediate medical intervention. Six hr after each blinded morning dose the subjects were asked to estimate the dose level, for conformation of effectiveness of the blinding procedure.

Statistical analysis: demographic and pharmacokinetic data. Differences between inhalers and the intravenous group regarding demographic and pharmacodynamic features were tested with Pearson's correlation tests. Differences in t 1/2 of heroin or its metabolites, between the inhalation and the intravenous group, were tested using independent sample t‐tests. Pharmacokinetic‐pharmacodynamic relationships were studied univariately with linear regression analysis. Significant co‐variables were entered in repeated measurements ANOVA or multiple linear regression models.

The inter‐subject variability of the pharmacokinetic parameters, were calculated by dividing the standard deviations by the means. The intra‐individual variability of the AUC's was based on two AUC outcomes pro person, following regular heroin dosages. The standard error of bioavailability F was calculated by adding the sum of co‐variances of the numerator (inhalation AUC's + intravenous dosages) and the sum of co‐variances of the denominator (intravenous AUC's + inhalation dosages).

Statistical analysis: pharmacodynamic data. Differences between the baseline (E0), and post heroin or methadone administration levels of pharmacodynamic parameters (Et), were tested with paired sample t‐tests. The mean percentage of change, after heroin administration, was calculated as ((Et‐E0)/E0)*100. When the impact of intravenous use was compared to effects of inhalation, independent sample t‐tests were used.

Pharmacodynamic measurements were also analysed with repeated measurements ANOVA, with time‐points of assessments and dose‐level as within factors and the way of heroin administration and co‐administered medication as between factors. Methadone dose or methadone plasma trough concentrations, were introduced in the repeated measurements ANOVA model as co‐variables. Measurements of the subjective VAS and PSQ, taken during the first day of the trial, when regular heroin dosages were administered un‐blinded, were not taken into account during analysis.

All statistical calculations were performed with SPSS, version 11 (Statistical Package Service and Solutions Inc., Richmond, CA, USA. 2001). All tests for significance were two tailed and the P‐value required for significance was set at 0.05.