Urinary Excretion and Paracetamol

Urinary Excretion and ParacetamolInvestigateurinary excretion of paracetamol in man.Paracetamol, known as acetaminophen in the USA, is one of the most commonly used analgesic and antipyretic medicines available over-the-counter. Its common name derives from the full chemical name para-acetyl-amino-phenol, with the chemical beula C8H9NO2 and amolecular weight of 151.17.Paracetamol does not have anysignificant anti-inflammatory action and therefore cannot be accuratelydescribed as a non-steroidal anti-inflammatory drug (NSAID), as was oncethought. Its mechanism of action is still poorly understood but slightly studieshave suggested that it inhibits a variant of the cyclo-oxygenase enzyme COX-1, which has been designated COX-3 (Swierkosz et al. 2002). Paracetamol actsmainly in the central nervous system and endothelial cells, rather than inplatelets and immune cells. Boutaud and colleagues (2002) hypothesised thatthis may be explained by the high takes of peroxides found in the l atter cell types, which inhibit the action of paracetamol. There has been some debate on thesubject, with other researchers proposing an inhibitory action against COX-2(Graham Scott 2005). Further research is required to fully elucidate the mechanism of action at the molecular level.Metabolism and excretionFollowing oral administration and absorption from the gastrointestinal tract, paracetamol enters the blood and is distributed throughout the body. It is metabolised by enzymes in the hepatocytes of the liver and the majority is converted to idle metabolites by conjugation with sulphate or glucuronide. This is then filtered out of theblood by the kidneys and into the urine, via active renal vasiform secretion. Asmall portion of paracetamol remains unaltered and passes into the urine viaglomerular filtration and passive absorption (Morris Levy 1984). Thereis also a small proportion of the paracetamol that is metabolised by the thecytochrome P450 system, which results in the fo rmation of cysteine or glutathioneconjugates and mercapturic acid conjugates (figure 2). These products ofoxidative metabolism are also excreted renally (Andrews et al. 1976).Paracetamol has a low therapeuticindex, so the therapeutic process is very close to the toxic dose. Toxicity canoccur following a single large dose (10g) or with chronic lower doses(4-5g/d) and is ordinarily seen as hepatotoxicity, which can result in deathwithin several days (Wikipedia).Toxicity occurs when the enzymesresponsible for catalysing sulphate and glucuronide conjugation becomesaturated, forcing metabolism to be increasingly dependent upon the cytochromeP450 system. This results in formation of a toxic metabolite,N-acetyl-p-benzo-quinone imine (NAPQI), which is normally mopped up by bindingto the sulphydryl group of glutathione to form inactive conjugates andmercapturic acid. Toxicity occurs when the glutathione supply becomes exhaustedand NAPQI binds indiscriminately to molecules within the cell, such asmembranes, to cause cell damage and death, seen as acute hepatic necrosis.1)Major parcel for normal metabolism2)Minor pathway via cytochrome P450 system receives toxic metabolite (NAPQI),shown in red. Normally this is detoxified by binding to glutathione.3) Toxicity occurs when pathways 1 and 2 are overloadedand NAPQI binds to molecules of the cell, causing damage.Modifiedfrom Rang et al. 1995.Aim of examineThe aim of this experiment is toinvestigate the renal excretion of paracetamol, by measuring the levels ofparacetamol metabolites in human urine over 6 hours following an oral dose of500mg. The total excretion leave behind be assessed using the spectrophotometricmethod. From this data the elimination rate constant (KE) and thehalf- support (T1/2) will be calculated. Qualitative analysis of thevarious metabolites will be conducted using appropriate chemical identificationtechniques.METHODA standard stock solution ofparacetamol was prepared at 1mg/cm3 and dilutions were made to givea range of known concentrations. 1 cm3 of the paracetamol solutionwas added to 1 cm3 blank urine and 4 cm3 4M HCl, andmixed thoroughly. A blank duplicate was also prepared, using water instead ofurine. After an hour in a boiling water bath the tubes were cooled and wateradded, up to 10 cm3. 1 cm3 of this hydrolysed urinesolution was added to 10 cm3of colour forming solution, mixed and allowed to stand for40 minutes. The absorbance of each solution was measured, using thespectrophotometer, zeroing the instrument using the drug free urine stress inbetween solutions. This produced the readings for the calibration curve. Thecollected timed urine auditions were then processed in the same way, adding 1 cm3water instead of paracetamol solution.RESULTS AND DISCUSSIONKnown concentrations of paracetamolunderwent spectrophotometry to measure the absorbance at 620nm. These resultswere used to produce a calibration curve (figure 3). The timed urine sampleswere then analysed follow ing the same protocol and the absorbance at 620nm wasused, in conjunction with the calibration curve to bump the concentrationof paracetamol in the urine. Unfortunately, half of the samples producedabsorbances outside the range of the calibration curve. Because this curve isnon-linear, extrapolation and dilution cannot be used to accurately deduce theconcentration of paracetamol in the urine. For the purposes of this report theconcentration for these samples has been declared as greater than 800ug/cm3.This is not very satisfactory and further experiments must be done to extendthe range of the calibration curve to the maximum absorbancy of the timedsamples. The values of KE and T1/2 have been calculatedto picture the procedure, but are inaccurate and will need revising onceaccurate concentrations have been established form the calibration curve. hedge 1Timed urine sampleMean absorbance 620nmConc. ug/cm3Vol. Urine (ml)Total drug (ug of paracetamol)Excretion rate mg/h0000001 hour0.25 619224547040472 hours1.9188005040000403 hours1.769800383040030.44 hours1.0288005544000445 hours0.3492461353321033.26 hours0.2551921603072030.7Table 1 contains the absorbanceresults of the timed urine samples and the deduced concentration of paracetamolin the urine, as well as the hourly excretion rate. The total come in ofparacetamol excreted over the 6 hour consequence was 225.3mg, which is 45% of theorally administered dose. Due to problems discussed above, this is anunderestimate of the true percentage of dose excreted renally, which has beenfound to be 55-70% by other studies (Steventon et al. 1996).When log of the excretion rate(equivalent to total drug excreted per hour) is plotted against time, a linearplot should be achieved, from which KE can be estimated.The slope of this straight lineequates to KE /2.303, which gives a value for KE of0.094. Using the formula T1/2 =0.692/ KE , the valueof T1/2 = 7.36 hours.This states that it takes the body7.36 hours to excrete half of the drug administered. This is longer than the1-4 hours usually quoted for paracetamol (Rang et al. 1995), and is notsurprising given the underestimation of the paracetamol urine concentration.With proper calibration, this would be expected to decrease to nearer thepreviously found results.There were no results for thequalitative studies for metabolite composition, but it would be expected thatsulphate and glucuronide conjugates would make believe the majority of the sample,with a smaller quantity of unchanged paracetamol, cysteine/glutathione andmercapturic acid metabolites.These results only represent oneindividual on one day and replications of this experiment are crucial.Nutritional status, recent alcoholic beverage consumption, ethnic background, concurrentdrug usage and illness must all be taken into account as factors that mayaffect paracetamol metabolism and excretion (Riordan Williams 2002, Patel Tang 1992).Further analysis of paracetamolexcretion.Hepatotoxicity and drug interactionsTable 2 shows how concurrent use of phenobarbital, ananti-epileptic drug, can increase the severity of liver damage caused byparacetamol administration and its subsequent metabolism.Table 2 Effect of purple heart onparacetamol induced hepatotoxicityTreatmentDose of Paracetamol (mg/kg) Severity of liver necrosisNone 375 1-2+Phenobarbital 375 2-4+_________This occurs due to metabolism ofphenobarbital by enzymes of the P450 cytochrome system, which results inupregulation of their production. As explained in the introduction (see fig.2), P450 enzymes also metabolise paracetamol, to form the toxic metaboliteNAPQI. This is normally a minor pathway but as the amount of P450 enzymesavailable increases, the activity of this pathway also increases. This resultsin a larger than normal amount of NAPQI, which is mopped up and inactivated byglutathione. Glutathione supplies will eventually run out, which occurs soonerif the person is malnourished. When this happens the toxic metaboli te binds tocell components, causing necrosis. To prevent this occurring, such as in casesof overdose, N-acetylcysteine can be given (Routledge et al. 1998), which isrequired for glutathione implication and helps to boost it. This allows agreater amount of the toxic metabolite to be mopped up and reduces cell damage..Paracetamol metabolism following hepatotoxicityTable 3Plasmaparacetamolconcentrations(ug/cm3)Patients Plasmaparacetamol 4 hrs after 12hrsafterHalf life (h) ingestion ingestion_______________________________________________________________noliver damage (18) 2.9 +/= 0.3 163 +/=20 29.5 +/=6liverdamage (23) 7.2+/= 0.7 296 +/= 26 124 +/=22___Table 3 shows that, in a study, theability of patients with liver damage to eliminate paracetamol from the bloodis much decreased, compared to brawny people. This is seen by the prolongedhalf-life and the high levels of paracetamol in the plasma. The plasma leveldoes come down by 12 hrs, which indicates that there is enough functional liverreserve to metabolise some of the drug, but the level is still very high. Toascertain whether it is just conjugation that is affected, or whether all thepathways are affected equally it would be necessary to quantify the levels ofdifferent metabolites in the blood and urine. As conjugation is responsiblefor the majority of metabolism, damage to all systems will still show up asaffecting conjugation the most.In theory reduced clearance of asubstance is useful for monitor the severity of liver damage, but in thecase of paracetamol it would be unwise as it could potentiate the hepatotoxiceffects and worsen the liver condition. It is also unnecessary as there arealready a number of reliable blood tests for liver function and damage.REFERENCESAndrews, R. S., Bond, C. C., Burnett, J., Saunders, A. Watson, K. 1976 Isolation and identification of paracetamol metabolites. J Int Med Res 4,34-9.Boutaud, O., Aronoff, D. M., Richardson, J. H., Marnett, L. J. Oates, J. A. 2002 Determinants of the cellular specificity of acetaminophen as an inhibitor of prostaglandin H(2) synthases. Proc Natl Acad Sci U S A 99,7130-5.Graham, G. G. Scott, K. F. 2005 Mechanism of action of paracetamol. Am J Ther 12, 46-55.Morris, M. E. Levy, G. 1984 Renal clearance and serum protein binding of acetaminophen and its major conjugates in humans. J Pharm Sci 73, 1038-41.Patel, M., Tang, B. K. Kalow, W. 1992 Variability of acetaminophen metabolism in Caucasians and Orientals. Pharmacogenetics 2, 38-45.Rang, H. P., Dale, M.M., Ritter, J.M. 1995 Pharmacology Churchill Livingstone.Riordan, S. M. Williams, R. 2002 Alcohol exposure and paracetamol-induced hepatotoxicity. Addict Biol 7, 191-206.Routledge, P., Vale, J. A., Bateman, D. N., Johnston, G. D., Jones, A., Judd, A., Thomas, S., Volans, G., Prescott, L. F. Proudfoot, A. 1998 Paracetamol (acetaminophen) poisoning. No need to change current guidelines to accident departments. Bmj 317, 1609-10.Steventon, G. B., Mitchell, S. C. Waring, R . H. 1996 Human metabolism of paracetamol (acetaminophen) at different dose levels. Drug Metabol Drug Interact 13, 111-7.Swierkosz, T. A., Jordan, L., McBride, M., McGough, K., Devlin, J. Botting, R. M. 2002 Actions of paracetamol on cyclooxygenases in tissue and cell homogenates of mouse and rabbit. Med Sci Monit 8, BR496-503.http//en.wikipedia.org/wiki/Paracetamol.