Relative bioavailability of sodium cromoglycate to the lung following inhalation, using urinary excretion

Abstract

Aims

To determine if a urinary excretion method, previously described for salbutamol, could also indicate the relative bioavailability of sodium cromoglycate to the lung following inhalation from a metered dose inhaler.

Method

Inhaled (INH), inhaled+oral charcoal (INHC), oral (ORAL) and oral+oral charcoal (ORALC) 20 mg doses of sodium cromoglycate were given via a randomised cross-over design to 11 healthy volunteers trained on how to use a metered dose inhaler. Urine samples were collected at 0.0, 0.5, 1.0 and up to 24 h post dosing and the sodium cromoglycate urinary concentration was measured using a high performance liquid chromatographic method.

Results

No sodium cromoglycate was detected in the urine up to 24 h following ORALC dosing. A mean (s.d.) of 3.6 (4.3) μg, 10.4 (10.9) μg and 83.7 (71.1) μg of the ORAL dose was excreted, in the urine, during the 0.5, 1.0 and 24 h post dose collection periods, respectively. Following INH dosing, the renal excretion was significantly higher (P < 0.01) with 32.9 (14.5) μg, 61.2 (28.3) μg and 305.6 (82.3) μg excreted, respectively. The SCG excreted at 0.5, 1.0 and 24 h collection periods following INHC dosing were 26.3 (8.4) μg, 49.3 (18.1) μg and 184.9 (98.4) μg, respectively. There was no significant difference between the excretion rate of sodium cromoglycate following INHC when compared with INH dosing in the first 0.5 and 1.0 h.

Conclusions

The urinary excretion of sodium cromoglycate in the first 0.5 h post inhalation can be used to compare the relative lung deposition of two inhaled products or of the same product using different inhalation techniques. This represents the relative bioavailability of sodium cromoglycate to the lung following inhalation. Similar 24 h urinary excretion of sodium cromoglycate can be use to compare the total dose delivered to the body from two different inhalation products/inhalation methods. This represents the relative bioavailability of sodium cromoglycate to the body following inhalation. Because of the lack of difference between the INH and INHC in the first 0.5 h, the use of activated charcoal is not necessary when this method is used to compare the relative lung bioavailability of different products or techniques.

Introduction

Administration of a drug by inhalation for the treatment of asthma provides both a rapid onset of action and a reduction (or even avoidance) of systemic side effects [1]. Currently, most of the drugs commonly used for treating reversible airflow obstruction are available as metered dose inhalers (MDI), dry powder inhalers (DPI) or nebulizers. Following inhalation, greater than 80% of the dose is swallowed and reaches the gastrointestinal tract, thus less than 20% is deposited into the airways and only the pulmonary absorbed fraction is responsible for the pharmacological effect [2]. Application of traditional pharmacokinetic methods to lung deposition studies are difficult, because the doses administered are small and the resulting systemic drug concentrations are either due to the swallowed and inhaled fraction [3] or so low as to be difficult to assay accurately or reproducibly [4].

Pulmonary deposition following inhalation may be studied by gamma scintigraphy [5]. Despite the specificity and sensitivity of this method to determine total and regional lung deposition, it is two-dimensional and the radioactive labelling could change the formulation in question or could move to other ingredients in the formulation. In addition, radioactive methods expose patient or volunteers to radiation (especially at areas of impaction in the airways), and the experimental set-up is technically complicated and costly.

Pharmacokinetic methods to evaluate the relative lung bioavailability of inhaled drugs have used plasma salbutamol concentrations over the first 20 min post inhalation [6], urinary terbutaline excretion with administration of oral charcoal to block oral absorption [7] and the amount of salbutamol excreted in the urine in the first 30 min post inhalation [8]. The latter study demonstrated that following oral administration of salbutamol, negligible amounts of salbutamol are excreted in the urine during the first 30 min post dose, and that significantly greater amounts (P < 0.01) are excreted 30 min post inhalation. Thus, the urinary excretion of salbutamol during the 30 min post inhalation is representative of the amount delivered to the lungs. This study also demonstrated that the 24 h urinary excretion of salbutamol and its metabolites reflects the total systemic bioavailability. This technique has been used to compare different inhalation methods and products [9, 10], to determine an optimal inhalation technique for MDIs [11], and provide useful information on the use of large volume spacers [12] and nebulizers [13].

Urinary excretion of sodium cromoglycate has been suggested to allow a direct estimate of the in vivo pulmonary deposition [14]. When taken orally sodium cromoglycate has negligible gastrointestinal absorption. Following inhalation, however, the fraction deposited into the lungs is well absorbed and rapidly eliminated unchanged in the urine and bile [15–18]. We have previously reported a reversed phase ion-pair high performance liquid chromatography (h.p.l.c.) assay for sodium cromoglycate [19] in the urine. Similar to the principle of the salbutamol method [8] the urinary excretion of sodium cromoglycate at 0.5 h following inhalation could be used to indicate the amount of drug reaching the respiratory tract and thus determine the relative bioavailability to the lungs following inhalation.

Methods

The study was divided into two parts. In the first part the dose of activated charcoal to completely block SCG gastrointestinal absorption was determined. This was achieved by identifying the urinary excretion of sodium cromoglycate (SCG) following the oral administration of a SCG solution taken with increasing oral doses of activated charcoal. In the second part, the pulmonary bioavailability and the combined pulmonary and gastrointestinal bioavailability of SCG given by a MDI were investigated.

Subjects

Healthy, non-smoking volunteers gave written informed consent (University of Bradford Ethics Committee Approval obtained) to take part in this study. For the first part of the study, three healthy volunteers (one female) swallowed 20 mg SCG in 20 ml water immediately after the oral administration of either 5, 10, 25 or 50g of activated charcoal (Carbamix, Penn Pharmaceuticals Ltd, Bucks, UK) in 250ml of water (shaken before taking). The order of the four study doses was randomized with a 7 day washout period between administrations. Urine samples were collected at 0.0, 0.5, 1.0, 2.0, 5.0 and 24 h post administration for analysis.

For the second part of the study 11 volunteers participated. Each was given, in randomised order at an interval of 7 days, either an oral dose of 20 mg SCG dissolved in 20 ml water (ORAL), 4×5 mg doses of SCG inhaled from a MDI (Intal®, Rhône-Poulenc Rorer, UK) (INH), 4×5 mg SCG inhaled from an Intal® MDI with concurrent oral administration of 25g activated charcoal (INHC) or an oral dose of 20 mg SCG solution (20 ml) together with the oral administration of 25g activated charcoal (ORALC). The oral dose of 25g activated charcoal with 250 ml water was selected as a result of the first part of the study. Water (250 ml) was swallowed after ORAL and inhaled dosing.

Healthy volunteers were chosen so that they were MDI naive and could be trained to use a standard inhalation technique. They were instructed to exhale to residual volume, actuate the MDI at the start of a slow deep inhalation (to total lung capacity), followed by a 10 s breath hold and then gently breathe out [20]. This manoeuvre was repeated every 30 s for each inhaled dose. Subjects emptied their bladder immediately before drug administration and urine samples were collected at 0.5, 1.0, 2.0, 5.0 and 24 h post dosing. The volume of urine excreted was recorded and an aliquot of each sample was frozen at −20° C prior to analysis.

Sample analysis

A previously described sensitive (h.p.l.c.) method was used to measure the concentration of SCG in the urine [19]. SCG was extracted from urine samples using solid-phase extraction (PH cartridge) then quantified in an ion-pair reversed phase h.p.l.c. system connected to u.v.-detector set at 238nm. The intra- and inter-assay precision for the assay were 8.3 and 13.6% at 0.25 μg ml⁻¹ and 3.0 and 8.6% at 2.0 μg ml⁻¹, respectively. The mean (s.d.) absolute recovery was 94.8 (0.6)% and the limit of determination was 0.1 μg ml⁻¹ (signal to noise ratio was greater than 10:1).

Statistical analysis

A paired Student’s t-test was used for comparisons. In addition the mean ratio (test/reference) with 95% confidence intervals were calculated, except for comparisons of the amounts of SCG excreted up to 0.5 and 1.0 h post oral dosing. For these the mean difference with 95% confidence interval were calculated because in some subjects no SCG was excreted in the urine during these two intervals.

Results

Three healthy (one female) volunteers participated in the first part of the study (age range 26–33 years). All urine samples were below the SCG lower limit of quantification (0.1 mg ml⁻¹) following the co-administration of 20 mg SCG with both 25 and 50g of activated charcoal (suspended in 250 ml water). A 25g oral charcoal dose was therefore, selected for the second part of the study because it completely blocked the SCG gastrointestinal absorption.

Eleven healthy volunteers (6 male), their mean (s.d.) age, weight and height, 31.5 (7.9) years, 75.4 (12.6) kg and 174.7 (8.7) cm, respectively, completed the second part of the study. No SCG was detected in all urine samples following ORALC. The mean (s.d.) urinary SCG excretion data for these 11 subjects following INH, INHC and ORAL administration are shown in Table 1 and Figure 1. These show that negligible amounts of sodium cromoglycate were excreted in the urine following oral administration during the first 0.5 and 1.0 h urine collection periods. There was no SCG detected from the urine samples of four subjects during the first 30 min post oral dosing. Overall less than 0.02% of the nominal sodium cromoglycate oral dose was excreted in the first 0.5 h, and 0.06% in the first hour. This fraction increased with time but did not exceed 0.5% in 24 h. Respective values following inhaled dosing were, 0.16, 0.31 and 1.53% of the nominal inhaled dose, respectively. Figure 2 shows that, the amount of SCG recovered at 30 min post dose was significantly higher (P < 0.001) after (a) INH and (b) INHC compared with ORAL. The mean difference (95% confidence intervals) of 0.0–0.5 h urinary drug excretion following INH and INHC administration compared with ORAL administration were 29.5 (18.6, 40,4) μg and 22.7 (16.0, 29.4) μg, respectively (P < 0.001). Similar values for the 0.0–1.0 h post dose were 51.2 (31.2, 71.2) μg and 38.9 (25.5, 52.3) μg, respectively (P < 0.001).

Table 1.

Mean (s.d.) Amount excreted (μg) and urinary excretion rate (μg h⁻¹) of SCG following INH, INHC and ORAL administration.

Figure 1.

Cumulative urinary excretion of SCG following 20 mg dosing via a MDI (INH), a MDI with simultaneous oral administration of 25g activated charcoal (INHC) and an oral solution (ORAL).

Figure 2.

Individual (n = 11) and mean (s.d.) amounts of SCG recovered following 20 mg dosing via (a) MDI (INH) vs swallowed as a solution (ORAL) and (b) a MDI and 25g oral activated charcoal (INHC) vs the oral solution (ORAL).

No significant difference was found between the amount of SCG excreted in the urine samples 0.0–0.5 and 0.0–1.0 h post dose following INH and INHC administration as shown in Figure 3 a and b, respectively. The mean ratio expressed as a percentage with (95% confidence intervals) between INH and INHC for the two sample periods were 126.0 (100.2, 151.9) and 123.5 (98.3, 148.6), respectively. For the collection intervals 1.0–2.0, 2.0–5.0 and 5.0–24 h there was a significant difference (P < 0.02) between INH and INHC in the amount recovered in the urine. When comparing the amount excreted in the urine following INH and ORAL dosing, a significant difference (P < 0.02) was found at all time intervals investigated, except for 5.0–24 h urine collection interval. This difference was also true for INHC with ORAL.

Figure 3.

Individual (n = 11) and mean (s.d.) comparison of the amounts of SCG recovered following inhalation of 20 mg via MDI (INH) and a MDI with oral administration of 25g activated charcoal (INHC) dosing (a) at 30 min and (b) 60 min urinary collection intervals.

The mean (s.d.) urinary excretion of SCG over the 24 h period post dosing of INH, INHC and ORAL were 305.6 (82.3), 184.9 (98.4) and 83.7 (71.1) μg, respectively. The mean ratio (95% confidence interval)% 24 h post dosing for INH vs ORAL, INHC vs ORAL and INH vs. INHC were 674.5 (358.0, 991.0)%, 420.6 (201.2, 640.0)% and 197.6 (140.1, 255.1)%, respectively.

Discussion

Although the first study only involved three subjects to determine the charcoal dose to block the gastrointestinal absorption of sodium cromoglycate the results from the subjects in the main (second) study confirm the decision to use 25g activated charcoal.

Statistically significantly more amounts of SCG were excreted in the urine following inhalation compared with oral administration during the first 5 h post dose. The ratios of the amounts of sodium cromoglycate recovered in the urine following oral and inhaled administration were 1:9.14, 1:5.89 and 1:3.65 for the 0.5, 1.0 and 24 h urinary collection periods post dose, respectively. These data highlighted in Table 1 are due to the rapid and complete absorption of the sodium cromoglycate fraction which is deposited in the lungs, the slow and incomplete absorption from the gastrointestinal tract and the lag time before the drug appears in the systemic circulation and then urine post oral administration. This is similar to the salbutamol urinary excretion data following inhaled and oral administration presented by Hindle & Chrystyn [8]. Although the activated charcoal effectively blocked the gastrointestinal absorption of sodium cromoglycate, the amount excreted in the urine following inhaled administration with oral charcoal did not differ significantly from that following inhaled dosing during the first 0.5 and 1.0 h urine collection periods. The significant differences between the amounts of urinary sodium cromoglycate in samples taken more than 1 h post inhalation from the MDI and MDI with charcoal highlights the contribution of the orally absorbed fraction. Thus the amount of sodium cromoglycate excreted in the urine during the first 0.5 and 1.0 h following inhalation indicates the relative bioavailability of sodium cromoglycate to the lungs following inhalation and this index could be used to compare the in vivo lung deposition of inhaled sodium cromoglycate products or methods of inhalation.

The difference in the mean total amounts of sodium cromoglycate excreted in the urine following inhaled (305.6 μg) and inhaled with the oral charcoal plus the oral dose (INHC+ORAL; 184.9+83.7 = 268.6 μg) at 24 h post dosing could be attributed to the blockage, by activated charcoal, of sodium cromoglycate reabsorption through enterohepatic circulation. This is consistent to that reported after intravenous dosing where a significant fraction (up to 50%) of the dose of sodium cromoglycate is excreted through the biliary system [15, 16, 18]. Nevertheless the amount of sodium cromoglycate delivered to the body and excreted in the urine over the 24 h post inhalation period could be used, in crossover studies, to compare the emitted dose via different inhalation products or techniques. The relative bioavailability of sodium cromoglycate to the body following inhalation compared to the oral dose was >600% because the majority of the sodium cromoglycate excreted in the urine has been absorbed by the pulmonary route.

In conclusion, the amount excreted following an inhaled SCG dose taken together with an oral dose of activated charcoal represents the pulmonary absorbed fraction. Because of the lack of difference between the inhaled (INH) and inhaled plus oral charcoal (INHC) in the first 30 min then the use of activated charcoal is not necessary when this method is used to compare between different products or techniques. The 24 h urinary excretion of SCG can be used to compare the delivered dose to the body from two different inhalation products or methods. Thus the urinary drug excretion technique which has already been considered to be the basis for a novel clinical equivalence test for inhalation products of salbutamol [5] and recently extended to nedocromil [21], can also be used for other water soluble inhaled drugs like sodium cromoglycate.