Week 1: Concepts in Clinical Pharmacology Flashcards
Dosing Instruction Abbreviations: BD
Twice Daily
Dosing Instruction Abbreviations: NOCTE
At night
Dosing Instruction Abbreviations: OD
Once a day
Dosing Instruction Abbreviations: QDS
Four times a day
Dosing Instruction Abbreviations: STAT
Single one off dose
Dosing Instruction Abbreviations: TDS
Three times a day
Pharmacokinetic Abbreviations: Cmax
Maximum concentration
Pharmacokinetic Abbreviations: Cp
plasma concentration
Pharmacokinetic Abbreviations: CpSS
Plasma concentration at steady state
Pharmacokinetic Abbreviations: CYP
Cytochrome P450
Pharmacokinetic Abbreviations: Tmax
Time to maximum concentration
Pharmacokinetic Abbreviations: Vd
Volume of Distribution
Administrative Route Abbreviations: IM
Intramuscular
Administrative Route Abbreviations: IV
Intravenous
Administrative Route Abbreviations: LD
Loading Dose
Administrative Route Abbreviations: PO
Oral
Administrative Route Abbreviations: PR
Per rectum
Administrative Route Abbreviations: SC
Subcutaneous
Administrative Route Abbreviations: SL
Sublingual
As a clinician, you are given the Apparent Volume of Distribution for three drugs. These have been tested in a clinical trial in a male patient, 35 yrs old of normal build. The values are given below. What real fluid compartments do you think these drugs are distribute between?
Drug 1: 0.12 L/kg
Drug 2: 0.26 L/kg
Drug 3: 3.4 L/kg
- 12 L/kg is 8.4 L - mainly plasma (e.g. insulin)
- 26 L/kg is approx 18 L- extracellular (e.g. penicillin)
- 4 L/kg is approx 238 L- mainly intracellular (morphine)
The idea is to show how changes in Vd broadly correlate to fluid compartments and their related set of binding sites.
Two female patients in their early twenties were being assessed for treatment with haloperidol for psychotic illness. Both patients were about 1.7m in height, but one patient weighed 46 kg, the other 92 kg.
Given the normal Vd for haloperidol is about 18L/kg, how would you expect this weight difference to affect its value?
How do you think this information would affect your initial dosing regime with haloperidol in these patients?
You may consider using a lower than normal dose to avoid the possibility of overdose in the underweight patient. In the obese patient you would probably start with normal doses and adjust from there.
The very high Vd for Haloperidol of 18L/Kg indicates that it is widely distributed throughout the rest of body tissues and it is comparatively lipid soluble. The patient with a weight of 92 kg is evidently more obese than the other patient. You would expect the value of Vd to increase due to its increased distribution in body adipose for this patient
For the patient with a weight of 46 kg she is underweight for her height is evidently less obese. You would then expect the value of Vd to decrease due to decreased volume of adipose tissue.
What factors referenced to three major systems would you expect to affect Clearance and why?
HRH
Hepatic – If you have hepatic deficit you would expect lowered clearance due to the reduction in available Phase I and II enzymes for metabolism
Renal –Similarly if renal clearance is affected either by reduced filtration, active OATP and OCTP transport and reduced tubular secretion then clearance will be affected adversely
Heart/Cardiac Output – If cardiac output is reduced then the rate of blood flow to the liver and kidneys also drop again meaning clearance will be reduced.
A 53 yr old man is admitted to hospital with a long history of alcohol abuse. He is severely jaundiced and has suspected renal failure. He is also underweight for his height.
Before you begin any drug therapy, what effect might you expect there to be on clearance of any drugs you might prescribe?
a. Alcohol abuse and jaundice point to liver failure. This would be expected to reduce clearance for many drugs therefore longer half life
b. Renal failure would also be expected to affect clearance half life
c. Reduced weight would also be expected to result in a lower volume of distribution for drugs partitioning in to body tissues
d. Therefore would have to take all this into account before starting therapy as each would affect doses of most drugs administered.
e. Problems with reduced clearance mean that systemic toxicity becomes more of a problem.
Drug Elimination Half Life – t1/2
The terms for the volume of distribution and clearance are used by clinicians to calculate the overall figure summarising first order elimination kinetics the half-life.
This is approximately
t1/2≈ 0.7 x Vd/CL