- Email: [email protected]
MEASUREMENT OF PLASMA-PARAQUAT CONCENTRATION
699 studies showing no clinically significant interaction with oral anticoagulants, may have contributed to the delay in the demonstration of potential interaction, since unpublished studies cannot be critically evaluated.
published
Departments of Physiology and Medicine, U niv ersity of Manitoba, and St. Boniface General
Hospital,
STEFAN A. CARTER
Winnipeg, Canada
MEASUREMENT OF PLASMA-PARAQUAT CONCENTRATION
poisoning by Dr Proudfoot and colleagues (Aug. 18, p. 330) emphasises the need for rapid and reliable estimation of plasma-paraquat concentrations, and implies that the easiest way to do this is by radioimmunoassayl or colorimetry.22 The radioimmunoassay involves several steps, including the use of an activated charcoal suspension to remove unbound radioactive paraquat, and the whole method requires skill and complex equipment which may not always be readily available SiR,—The paper
on
Gartnavel General Glasgow G12 0YN
Hospital,
JANUSZ KNEPIL
paraquat
in an emergency. The assay demands total dedication for 45-60 min from a laboratory worker who, on emergency duty in a district general hospital, is likely to be simultaneously committed to other, equally important and urgent analyses. Supplies of antibody and of radioactive paraquat are not generally available. Radioimmunoassay should be considered only when the circumstances permit batch analysis, where the sample is too small for alternative techniques to be practicable, or where assessment of plasma-paraquat is obscured by the presence of
diquat. The colorimetric method referred to is much simpler than the radioimmunoassay but emulsification can be a problem unless the extraction is done very gently over a long period. The centrifugation and separation steps will also hold up the result. A colorimetric method has been described3 which involves no more than the addition of two simple reagents with short, vigorous mixing, a short centrifugation step, and addition of a single colour-developing reagent before measurement of paraquat. The sensitivity of the method is comparable with that of the colorimetric procedure referred to above, and a single analysis takes less than 20 min. Published data on the spectrophotometric properties of paraquat are either inadequately defined4 or at variance with each other4.5 and with The following procedure is based on a paraquat my own dichloride preparation guaranteed to contain 72-4% paraquat, and donated by I.C.I. Central Toxicology Laboratory, Alderley Edge. The figures may prove useful as a basis for calibration of paraquat standards until definitive absorbance values for paraquat ion are agreed. Crystalline paraquat dichloride was dried at 120°C for 15 h in a vessel capped loosely with a disc of Whatman no. 41 filter paper. A precisely weighed portion of this paraquat preparation was dissolved in 50 mmol/1 sulphuric acid to form a solution of known paraquat concentration containing approximately 1 mg paraquat ion/dl. The solution was scanned against 50 mmol/1 sulphuric acid on a Pye ’Unicam SP8-100’ double beam, recording spectrophotometer. The absorbance maximum and the wavelength at which it occurred were noted. The procedure was repeated and the mean maximum absorbance of paraquat ion per mg per dl sulphuric acid solution was calculated. The procedure was repeated until the recalculated mean was constant to four decimal places. This figure was then used to calculate the molar absorbance of paraquat ion in 50 mmol/l sulphuric acid.
findings.
A consistent absorbance maximum for paraquat ion was 257nm. Eight measurements were required to establish a constant mean absorbance (A257.s) per mg paraquat ion per dl 1. Levitt T. Radioimmunoassay for paraquat. Lancet 1977, ii: 358. DR, Stewart MS. The rapid extraction of paraquat from plasma using an ion-pair technique. Clin Chim Acta 1979; 94: 241. 3. Knepil J. A short simple method for the determination of parquat in plasma. Clin Chim Acta 1977; 79: 387-90. 4. Ledwith A. Optical spectra. In: Autor AP, ed. Biochemical mechanisms of paraquat toxicity. New York: Academic Press, 1977: 233-34. 5 Clark EGC. Isolation and identification of drugs in pharmaceuticals, body fluids, and post mortem material. London: Pharmaceutical Press, 1974: 468. 2. Jarvie
50mrnol/1 sulphuric acid of 1-111. The range of concentrations of paraquat ion tested was 0.7 to 1.385 mg/dl and a mean molar absorbance was calculated to be 20 694 1 mol-I cm-l. The regression line between paraquat-ion concentration and maximum absorbance had an intercept of zero. The calculated mean maximum absorbance per mg paraquat ion per dl sulphuric acid was identical to that described above. It seems therefore that the molar absorbance calculated above is a good approximation of the true molar absorbance for paraquat (1,1’-dimethyl-4,4’bipyridylium) ion in 50 mmol/1 H2SO4’
CIMETIDINE/DIAZEPAM INTERACTION SIR,-Dr Serlin and colleagues (Aug. 18, p. 318) describe an interaction between cimetidine and oral anticoagulants. We have been studying the pharmacokinetics of diazepam,aa commonly used drug which is often combined with cimetidine in the treatment of peptic ulceration and other gastric hyperacidity states. Four healthy, alcohol and drug free subjects received first a single intravenous bolus diazepam 0-1mg/kg. On a second occasion the same individuals were given four oral doses of 200 mg cimetidine in one day, and a fifth 200 mg dose was given half an hour before the intravenous injection of diazepam (0-11 mg/kg). In both experiments multiple blood-samples were taken for 72-96 h. Plasma concentrations of diazepam were measured by a specific and sensitive gas-chromatographic procedure2 and the pharmacokinetic data were analysed on a twocompartment open model. For statistical comparison the Student the table.
paired
t-test was
used. The results
are
summarised in
PHARMACOKINETICS OF DIAZEPAM WITH OR WITHOUT
CIMETIDINE PRETREATMENT
These data show that even short-term treatment with cimetidine significantly alters the pharmacokinetics of diazepam. The impairment of elimination, seen in the prolonged elimination half-life (T1I2()) and in the decreased total plasma clear-
ance (C,), suggests that, after multiple dosing, higher plasma levels of diazepam can be expected. The reduction in the hepatic elimination of diazepam is probably due to the inhibition of the microsomal drug metabolism by cimetidine, since in vitro and in vivo experiments with rats have demonstrated such an inhibitory effect. In addition, the decrease in the distribution volumes (Vdss or Vd) indicate, that the distributional pattern of diazepam has been also changed by cimetidine. Both pharmacokinetic alterations cause an increase in plasma-diazepam which could lead to a higher incidence of side-effects. Careful monitoring of drug action seems prudent if patients are treated with both diazepam and cimetidine. Dr Margarete Fischer-Bosch Institute for Clinical Pharmacology, 7000 Stuttgart 50, West Germany
U. KLOTZ V.-J. ANTTILA I. REIMANN
1. Klotz U. Clinical pharmacokinetics of diazepam and its biologically active metabolites. Klin Wschr 1978; 56: 895-904. 2. Klotz U, Avant GR, Hoyumpa A, Schenker S, Wilkinson GR. The effects of age and liver diseases on the disposition and elimination of diazepam in adult man. J Clin Invest 1975; 55: 347-59. 3. Riegelman S, Loo JCK, Rowland M. Shortcomings in pharmacokinetic analysis by conceiving the body to exhibit properties of a single compart-
ment. J Pharm Sci 1968; 57: 117-23. 4. Puurumen J, Pelkonen O. Cimetidine inhibits microsomal in the rat. Eur J Pharmacol 1979; 55: 335-36.
drug metabolism
published
Departments of Physiology and Medicine, U niv ersity of Manitoba, and St. Boniface General
Hospital,
STEFAN A. CARTER
Winnipeg, Canada
MEASUREMENT OF PLASMA-PARAQUAT CONCENTRATION
poisoning by Dr Proudfoot and colleagues (Aug. 18, p. 330) emphasises the need for rapid and reliable estimation of plasma-paraquat concentrations, and implies that the easiest way to do this is by radioimmunoassayl or colorimetry.22 The radioimmunoassay involves several steps, including the use of an activated charcoal suspension to remove unbound radioactive paraquat, and the whole method requires skill and complex equipment which may not always be readily available SiR,—The paper
on
Gartnavel General Glasgow G12 0YN
Hospital,
JANUSZ KNEPIL
paraquat
in an emergency. The assay demands total dedication for 45-60 min from a laboratory worker who, on emergency duty in a district general hospital, is likely to be simultaneously committed to other, equally important and urgent analyses. Supplies of antibody and of radioactive paraquat are not generally available. Radioimmunoassay should be considered only when the circumstances permit batch analysis, where the sample is too small for alternative techniques to be practicable, or where assessment of plasma-paraquat is obscured by the presence of
diquat. The colorimetric method referred to is much simpler than the radioimmunoassay but emulsification can be a problem unless the extraction is done very gently over a long period. The centrifugation and separation steps will also hold up the result. A colorimetric method has been described3 which involves no more than the addition of two simple reagents with short, vigorous mixing, a short centrifugation step, and addition of a single colour-developing reagent before measurement of paraquat. The sensitivity of the method is comparable with that of the colorimetric procedure referred to above, and a single analysis takes less than 20 min. Published data on the spectrophotometric properties of paraquat are either inadequately defined4 or at variance with each other4.5 and with The following procedure is based on a paraquat my own dichloride preparation guaranteed to contain 72-4% paraquat, and donated by I.C.I. Central Toxicology Laboratory, Alderley Edge. The figures may prove useful as a basis for calibration of paraquat standards until definitive absorbance values for paraquat ion are agreed. Crystalline paraquat dichloride was dried at 120°C for 15 h in a vessel capped loosely with a disc of Whatman no. 41 filter paper. A precisely weighed portion of this paraquat preparation was dissolved in 50 mmol/1 sulphuric acid to form a solution of known paraquat concentration containing approximately 1 mg paraquat ion/dl. The solution was scanned against 50 mmol/1 sulphuric acid on a Pye ’Unicam SP8-100’ double beam, recording spectrophotometer. The absorbance maximum and the wavelength at which it occurred were noted. The procedure was repeated and the mean maximum absorbance of paraquat ion per mg per dl sulphuric acid solution was calculated. The procedure was repeated until the recalculated mean was constant to four decimal places. This figure was then used to calculate the molar absorbance of paraquat ion in 50 mmol/l sulphuric acid.
findings.
A consistent absorbance maximum for paraquat ion was 257nm. Eight measurements were required to establish a constant mean absorbance (A257.s) per mg paraquat ion per dl 1. Levitt T. Radioimmunoassay for paraquat. Lancet 1977, ii: 358. DR, Stewart MS. The rapid extraction of paraquat from plasma using an ion-pair technique. Clin Chim Acta 1979; 94: 241. 3. Knepil J. A short simple method for the determination of parquat in plasma. Clin Chim Acta 1977; 79: 387-90. 4. Ledwith A. Optical spectra. In: Autor AP, ed. Biochemical mechanisms of paraquat toxicity. New York: Academic Press, 1977: 233-34. 5 Clark EGC. Isolation and identification of drugs in pharmaceuticals, body fluids, and post mortem material. London: Pharmaceutical Press, 1974: 468. 2. Jarvie
50mrnol/1 sulphuric acid of 1-111. The range of concentrations of paraquat ion tested was 0.7 to 1.385 mg/dl and a mean molar absorbance was calculated to be 20 694 1 mol-I cm-l. The regression line between paraquat-ion concentration and maximum absorbance had an intercept of zero. The calculated mean maximum absorbance per mg paraquat ion per dl sulphuric acid was identical to that described above. It seems therefore that the molar absorbance calculated above is a good approximation of the true molar absorbance for paraquat (1,1’-dimethyl-4,4’bipyridylium) ion in 50 mmol/1 H2SO4’
CIMETIDINE/DIAZEPAM INTERACTION SIR,-Dr Serlin and colleagues (Aug. 18, p. 318) describe an interaction between cimetidine and oral anticoagulants. We have been studying the pharmacokinetics of diazepam,aa commonly used drug which is often combined with cimetidine in the treatment of peptic ulceration and other gastric hyperacidity states. Four healthy, alcohol and drug free subjects received first a single intravenous bolus diazepam 0-1mg/kg. On a second occasion the same individuals were given four oral doses of 200 mg cimetidine in one day, and a fifth 200 mg dose was given half an hour before the intravenous injection of diazepam (0-11 mg/kg). In both experiments multiple blood-samples were taken for 72-96 h. Plasma concentrations of diazepam were measured by a specific and sensitive gas-chromatographic procedure2 and the pharmacokinetic data were analysed on a twocompartment open model. For statistical comparison the Student the table.
paired
t-test was
used. The results
are
summarised in
PHARMACOKINETICS OF DIAZEPAM WITH OR WITHOUT
CIMETIDINE PRETREATMENT
These data show that even short-term treatment with cimetidine significantly alters the pharmacokinetics of diazepam. The impairment of elimination, seen in the prolonged elimination half-life (T1I2()) and in the decreased total plasma clear-
ance (C,), suggests that, after multiple dosing, higher plasma levels of diazepam can be expected. The reduction in the hepatic elimination of diazepam is probably due to the inhibition of the microsomal drug metabolism by cimetidine, since in vitro and in vivo experiments with rats have demonstrated such an inhibitory effect. In addition, the decrease in the distribution volumes (Vdss or Vd) indicate, that the distributional pattern of diazepam has been also changed by cimetidine. Both pharmacokinetic alterations cause an increase in plasma-diazepam which could lead to a higher incidence of side-effects. Careful monitoring of drug action seems prudent if patients are treated with both diazepam and cimetidine. Dr Margarete Fischer-Bosch Institute for Clinical Pharmacology, 7000 Stuttgart 50, West Germany
U. KLOTZ V.-J. ANTTILA I. REIMANN
1. Klotz U. Clinical pharmacokinetics of diazepam and its biologically active metabolites. Klin Wschr 1978; 56: 895-904. 2. Klotz U, Avant GR, Hoyumpa A, Schenker S, Wilkinson GR. The effects of age and liver diseases on the disposition and elimination of diazepam in adult man. J Clin Invest 1975; 55: 347-59. 3. Riegelman S, Loo JCK, Rowland M. Shortcomings in pharmacokinetic analysis by conceiving the body to exhibit properties of a single compart-
ment. J Pharm Sci 1968; 57: 117-23. 4. Puurumen J, Pelkonen O. Cimetidine inhibits microsomal in the rat. Eur J Pharmacol 1979; 55: 335-36.
drug metabolism