Accelerated Stability Testing

24/11/2023 1 2 1. Understand and recall Phase I and Phase II metabolic reactions. 2. Recall enzyme inducers and inhibitors and understand their effect on metabolism. 3. Understand and recall the kinetics of drug absorption and distribution (calculations) Bioavailability, distribution, volume of distribution, steady state concentration, plasma clearance, first order kinetics, drug excretion. 2 1 24/11/2023 3 What is metabolism and why do we need it? Enzyme mediated conversion of a lipophilic compound into a more water soluble one. Drugs must be metabolised for efficient excretion once they have produced their effect otherwise their beneficial effects will become toxic What phases does it involve? Phase I: cytochrome P450: oxidation, hydrolysis, reduction. Phase II: glucuronidation, methylation, acetylation. sulphation, glutathione conjugation, 3 4 4 2 24/11/2023 5 • Some factors will induce drug metabolising enzymes, so the drug is metabolised more quickly (i.e. drug is less effective) e.g. Barbiturates, like phenobarbital, stimulates cytochrome P450 and accelerates the metabolism of warfarin making it less effective. • Enzyme Inducers - Crap GPsss (induce me to madness) Carbamazepine (anti-epileptic) Rifampicin (antibiotic, particularly used for tuberculosis) Alcohol - chronic consumption Phenytoin (anti-epileptic) Griseofulvin (anti-fungal) Phenobarbital (anti-epileptic barbiturate) Sulphonylureas (anti-diabetic drugs, e.g. gliclazide), St John's Wort (herbal remedy used for depression) and Smoking 5 6 Some factors will inhibit drug metabolising enzymes, increasing drug accumulation and increasing the risk of toxicity. Inhibitors - GSickFaces.com Grapefruit juice Sodium Valproate (anti-epileptic, also used for bipolar disorder) Isoniazid (used for tuberculosis) Cimetidine (a H2 histamine receptor blocker) Ketoconazole & fluconazole (anti-fungals) Fluoxetine (a selective serotonin reuptake inhibitor) Amiodarone and Alcohol – binge Chloramphenicol (broad spectrum antibiotic) Erythromycin & clarithromycin (macrolide antibiotics) Sulphonamides (antibiotics) Ciprofloxacin (a quinolone antibiotic, which can cause tendonitis and tendon rupture) Omeprazole (a proton-pump inhibitor) Metronidazole (antibiotic effective against anaerobes) 6 3 24/11/2023 7 a) Do you continue to dispense this medication or do you query this with the pharmacist? Give a reason for your answer. Query with the pharmacist. Drug-Drug Interaction. Grapefruit juice is a cytochrome P450 enzyme inhibitor. It will inhibit the metabolism of simvastatin. Simvastatin will accumulate in the body, increasing the risk of toxicity. Simvastatin causes muscle toxicity (rhabdomyolysis). Muscle ache is a sign of rhabdomyolysis. 7 8 b) What should be measured and why? If muscle pain, weakness, or cramps occur during treatment, creatine kinase concentrations should be measured. Creatine kinase (CK) levels are markedly elevated with rhabdomyolysis and that will help decide whether the simvastatin should be stopped (if the concentration is more than 5 times the ULN (in the absence of strenuous exercise) 8 4 24/11/2023 9 a) What does WWHAM stand for? W Who is the patient W What are the symptoms H How long have the symptoms been present A Action taken M Medication being taken b)This is a drug-drug interaction. St John’s wort is an enzyme inducer. It will induce the metabolism of levonorgestrel. Decreasing the efficacy of levonorgestrel. As a result, patients may experience breakthrough bleeding and potential contraceptive failure. St John’s wort should not be taken at the same time as an oral contraceptive pill or patients should use alternative methods of contraception 9 10 Drug combination Consequence of combination Explanation for interaction Warfarin + metronidazole Increased anticoagulant effect of warfarin Metronidazole is a cyp450 enzyme inhibitor-reduces metabolism of warfarin, warfarin accumulates, increased warfarin toxicity (increased risk of bleeding) Atorvastatin + rifampicin Rifampicin markedly decreases the exposure to Atorvastatin Rifampicin is a cyp450 enzyme inducer-increasing metabolism of atorvastatin (reducing therapeutic effect) Clarithromycin + amlodipine Clarithromycin is predicted to increase the exposure to Amlodipine. Clarithromycin is a cyp450 enzyme inhibitor-reduces metabolism of amlodipine, amlodipine accumulates. 10 5 24/11/2023 Recap - Pharmacokinetics 11 Bioavailability (F) F= amount of drug absorbed in systemic circulation following oral administration amount of drug absorbed when thesame dose is administered intravenously Volume of Distribution (Vd) Volume of Distribution (Vd) = amount of drug in body concentration of drug in blood • The levels of drug present at steady state concentration depend on the size of each dose given as well as the frequency of dosing. • The area under the curve (AUC) represents the total amount of drug available in the blood supply during the dosing regimen. • Half-life (T0.5 ) - The time it takes for the plasma drug concentration to fall to half its original value. • Plasma clearance (Clp) of drugs is dependent on F, t0.5, Vd and initial dose. 11 Recap – Pharmacokinetics formulae kel = 0.693 t0.5 DOSE = C0 x Vd Clp = F x DOSE AUCoral Clp = kel x Vd AUC0- = C0 kel F = AUCoral AUCi.v. AUC0- = DOSE Clp 12 Css = F x DOSE Clp x t interval Css = rate of infusion Clp 12 6 24/11/2023 13 13 14 What percentage of the dose of an intravenously administered drug remains in the body following 4 half-lives? Assume first-order kinetics. Give your answer to 2 dp. Answer: 6.25% Working out: Started off with 100% First half-life 50% Second half-life 25% Third half-life 12.5% Fourth half-life 6.25% 14 7 24/11/2023 15 Determine the t1/2 (hr) of a drug, to 1 dp, if is eliminated from the body at a rate of 10% per hr Answer: 6.9 hr Working out: Kel = 0.693 t1/2 t1/2 = 0.693 = 0.693 = 6.9 hr Kel 0.10 15 16 Assuming first order kinetics, how much of the drug (mg) will remain in the body 15 hours after intravenous administration, if the drug has a half-life of 5 hours and 200 mg was originally administered? Answer: 25 mg Working out: Initially administered First half life (5 hrs) Second half life (10 hrs) Third half-life (15 hrs) 200 mg 100 mg 50 mg 25 mg 16 8 24/11/2023 17 200 mg of a drug was administered intravenously. The drug plasma concentration was 10.4 µg/mL, calculate the apparent volume of distribution (L) to 3 sf Answer: 19.2 mL Working out: Vd= Dose = Cp 200 000 µg 10.4 µg/mL = 19230 mL (i.e. 19.2 L) 17 18 Using the following information, estimate the Cp (mg/L) of this drug to 3 sf: • Drug is administered via the IV route • 200 mg of the drug was administered • Patient weighs 60 Kg • Vd is 2.3 L/Kg Answer: 1.45 mg/L Working out: Vd= Dose Cp Cp = Dose = 200 mg Vd (2.3 x 60) L = 1.45 mg/L 18 9 24/11/2023 19 19 20 Please note: This is a semi-log plot, therefore you DO NOT need to log the plasma drug concentrations. From the provided semi-log plot of the data, determine the concentration at T0 (Cp0). 7.75 µgcm-3 20 10 24/11/2023 21 7.75 µgcm-3 t0.5, Is the time taken for the concentration to drop by half. i.e. divide the plasma concentration by 2 ie 3.8 µgcm-3 and read off time corresponding to this value (55h) 21 22 iii) Kel from formula kel = 0.693 = 0.693/55 = 0.0126h-1 t0.5 iv) Volume of distribution from the formula DOSE = C0 x Vd thus Vd = 40000 µg/7.75 µgcm -3 = 5161 cm3 mL to L is divided by 1000 i.e. 5.1 litres Note dose given in first paragraph as 40 mg but units of y axis is not in mg v) Plasma clearance = Clp = kel x Vd = 0.0126 x 5.1 = 0.064Lh-1 vi) AUC = AUC0- = C0/ kel = 7.75 µgcm-3 / 0.0126h-1 = 615 µgcm-3.h b) Comment on the values obtained for half-life and volume of distribution. What do these values indicate with regard to the distribution of the drug in the body and its metabolism? Half-life of 55h is long – slow elimination from the body. Metabolism is slow. Accumulation likely, therefore, risk of overdose. Vd of 5.1 litres is low – this indicates poor distribution around the body (probably restricted to distribution in the blood) 22 11