A-Textbook-of-Clinical-Pharmacology-and-Therapeutics-5th-edition

36 EFFECTS OF DISEASE ON DRUG DISPOSITION
and other plasma proteins. This alters the pharmacokinetics of
many drugs, but is seldom clinically important. Phenytoin
is an exception, because therapy is guided by plasma concentration
and routine analytical methods detect total (bound
and free) drug. In renal impairment, phenytoin protein binding
is reduced by competition with accumulated molecules
normally cleared by the kidney and which bind to the same
albumin drug-binding site as phenytoin. Thus, for any measured
phenytoin concentration, free (active) drug is increased
compared to a subject with normal renal function and the
same measured total concentration. The therapeutic range
therefore has to be adjusted to lower values in patients with
renal impairment, as otherwise doses will be selected that
cause toxicity.
Tissue binding of digoxin is reduced in patients with
impaired renal function, resulting in a lower volume of distribution
than in healthy subjects. A reduced loading dose of
digoxin is therefore appropriate in such patients, although the
effect of reduced glomerular filtration on digoxin clearance is
even more important, necessitating a reduced maintenance
dose, as described below.
The blood–brain barrier is more permeable to drugs in
uraemia. This can result in increased access of drugs to the
central nervous system, an effect that is believed to contribute
to the increased incidence of confusion caused by cimetidine,
ranitidine and famotidine in patients with renal failure.
METABOLISM
Metabolism of several drugs is reduced in renal failure. These
include drugs that undergo phase I metabolism by CYP3A4.
Drugs that are mainly metabolized by phase II drug metabolism
are less affected, although conversion of sulindac to its
active sulphide metabolite is impaired in renal failure, as is the
hepatic conjugation of metoclopramide with glucuronide and
sulphate.
RENAL EXCRETION
Glomerular filtration and tubular secretion of drugs usually
fall in step with one another in patients with renal impairment.
Drug excretion is directly related to glomerular filtration
rate (GFR). Some estimate of GFR (eGFR) is therefore
essential when deciding on an appropriate dose regimen.
Serum creatinine concentration adjusted for age permits calculation
of an estimate of GFR per 1.73 m 2 body surface area.
This is now provided by most chemical pathology laboratories,
and is useful in many situations. Alternatively, Figure 7.2
shows a nomogram given plasma creatinine, age, sex and
body weight and is useful when a patient is markedly over- or
underweight. The main limitation of such estimates is that
they are misleading if GFR is changing rapidly as in acute
renal failure. (Imagine that a patient with normal serum creatinine
undergoes bilateral nephrectomy: an hour later, his serum
creatinine would still be normal, but his GFR would be zero.
Creatinine would rise gradually over the next few days as it continued
to be produced in his body but was not cleared.) A normal
creatinine level therefore does not mean that usual doses
can be assumed to be safe in a patient who is acutely unwell.
eGFR is used to adjust the dose regimen in patients with
some degree of chronic renal impairment for drugs with a low
therapeutic index that are eliminated mainly by renal excretion.
Dose adjustment must be considered for drugs for which
there is 50% elimination by renal excretion. The British
National Formulary tabulates drugs to be avoided or used
with caution in patients with renal failure. Common examples
are shown in Table 7.2.
Clearance
(ml/min)
150
130
110
100
90
80
70
60
50
40
30
20
Weight
(kg)
120
110
100
90
80
70
60
50
40
30
R
Age
(years)
25
35
45
25
55
65
45
35
55
75 65
85 75
95 85
95
Serum
creatinine
(mg/100 ml)
5.0
4.0
3.0
2.0
1.7
1.5
1.2
1.3
1.0
0.9
0.8 0.7
0.6
0.5
0.4
10
Figure 7.2: Nomogram for rapid evaluation of endogenous
creatinine clearance – with a ruler joining weight to age. Keep
ruler at crossing point on R, then move the right-hand side of the
ruler to the appropriate serum creatinine value and read off
clearance from the left-hand scale. To convert serum creatinine in
mol/L to mg/100mL, as is used on this scale, simply divide by
88.4. (Reproduced with permission from Siersbaek-Nielson K
et al. Lancet 1971; 1: 1133. © The Lancet Ltd.)
Table 7.2: Examples of drugs to be used with particular caution or avoided
in renal failure
Angiotensin-converting enzyme
inhibitors a
Aldosterone antagonists
Amphotericin
Ciprofloxacin
Digoxin
Low molecular weight heparin
NSAIDs
Angiotensin receptor
blockers a
Aminoglycosides
Atenolol
Cytotoxics
Lithium
Metformin
Methotrexate
a ACEI and ARB must be used with caution, but can slow progressive renal
impairment (see Chapter 28).