Pharmacokinetic changes in renal impairment and dosage considerations

2. • At the end of the lecture, the students must be able
to
– describe how renal diseases affect the pharmacokinetics
of drugs and clinical importance

3. Introduction
• The kidneys are the main organ by which drugs and their
metabolites are eliminated from the body and
• so in renal impairment dosing regimens of many drugs
must be adapted.
• Furthermore, the kidneys are a target for various kinds of
drug toxicity.

5. Renal impairment
• Use of drugs in patients with reduced renal function can give rise to
problems for several reasons
– Failure to excrete a drug or its metabolites may produce toxicity
– Sensitivity to some drugs is increased even if elimination is
unimpaired
– Many side effects are tolerated poorly by patients in renal failure
– Some drugs cease to be effective when renal function is reduced.

6. Principles of dosage adjustment in
renal impairment
It depends on
• Metabolism of the drug (whether the drug is
eliminated entirely by renal excretion or it is
totally metabolised by liver)
• Or how toxic the drug it self

7. For the drugs
with minor
or
no dose-related side effects
• Very precise modification of the dose regimen
is unnecessary and
• A simple dose reduction is sufficient

8. For drugs with small safety margin
• Dosage regimen should be based on GFR
• For those drugs, recommended regimens should be
used only as initial treatment guide
• Subsequent treatment must be adjusted according
to clinical response and plasma concentration.

9. Renal insufficiency and nephrotoxic
drugs
• Nephrotoxic drugs should, if possible, be avoided
in patients with renal disease because
consequences of nephrotoxicity are likely to be
more serious when the renal reserve is already
reduced.

10. Grades of renal impairment
Grades of renal impairment
Grade GFR Serum creatinine (approx.)
Mild 20-50 mL/minute 150-300 micromol/L
Moderate 10-20 mL/minute 300-700 micromol/L
Severe <10 mL/minute >700 micromol/L

11. • For drugs
– which need reduction in dose
– Which are potentially harmful
– Or are in effective
• in renal impairment,
• The above table is to be applied.

12. Renal function and age
• Renal function declines with age
• Many elderly patients have low GFR <50ml/min although it
may not indicated by increased serum creatinine.
• It is wise to assume that at least mild impairment of renal
function when prescribing in elderly.

14. • Drug absorption
– Uremia decreases GI absorption of drugs.
– Uremia alters first pass hepatic metabolism.
• ↓ first pass metabolism → ↑ amount of active drug in
systemic circulation →↑ bioavailability →↑ chance of
drug toxicity
– Drug bioavailability is more variable in patients
with impaired kidney function than in others.

15. • Drug distribution
– Impaired kidney function alters drug distribution.
– Eg. volume of distribution
• Oedema and ascities ↑ Vd of highly water soluble drugs →
usual doses of such drug given to oedematous patients may
lead to → low plasma level  therapeutic failure
• Dehydration or muscle wasting → ↓ Vd → usual dose can
result in unexpectedly high plasma concentration → toxicity
– Drug distribution can be altered by fluid removal
during dialysis.

16. • Drug metabolism
– Uremia slows the rate of phase I metabolism – reduction,
oxidation, hydrolysis.
– As well as, some phase II metabolism pathways
– And even the drug is metabolised in the liver, many of the
metabolites depend on the kidneys for their removal from
the body.
• Eg. morphine → active metabolites (excreted mainly from the urine)
• In patients with renal failure, morphine metabolised more slowly as
well as excretion is impaired → active metabolites ↑ → ↑ toxicity
(prolonged narcosis & respiratory depression).

17. • Drug excretion
– Renal handling of the drugs depends on
• Glomerular filtration
• Tubular secretion
• Tubular reabsorption
– GFR ↓ → renal clearance ↓→ plasma T½ ↑
• Drug level monitoring
• Measurement of plasma drug concentration is helpful in
assessing a particular dosage regimen in renal
insufficiency.
• Most important for drugs with narrow therapeutic rage
or effects that are difficult to measure.

18. Drug
GFR (mL/min)
HD CAPD
>50 10-50 <10
Acetaminophen 4hr 6hr 8hr As GFR <10 As GFR <10
Amlodipine 100% 100% 100% As normal
GFR
As normal
GFR
Atenolol 100%
24hr
50%
24hr
25%
24hr
As GFR <10 As GFR <10
HD – haemodialysis
CAPD – continuous ambulatory peritoneal dialysis
CRRT- continuous renal replacement therapy

19. References
• Mclntyre, CW, Shaw, S, Eldehni, MT. (2012). Prescribing
Drugs in Kidney Disease. In: Taal, MW, Chertow, GM,
MArsden, PA, Skorecki, K, Yu, ASL, Brenner, BM Brenner &
Rector's The Kidney. 9th ed. Philadelphia, USA: Elsevier .
2258-2289.
• British National Formulary, 63rd Edition.
• Singh, NP, Ganguli, A, Prakash, A. (Oct, 2003). Drug-induced
Kidney Diseases. Journal of Association of
Physicians India. 51 (5), 970-979.