CLINICAL PHARMACOLOGY
Mechanism Of Action
Atropine is a reversible antagonist of muscarine-like actions of acetyl-choline and is therefore
classified as an antimuscarinic agent. Atropine is relatively selective for muscarinic receptors. Its
potency at nicotinic receptors is much lower, and actions at non-muscarinic receptors are generally
undetectable clinically. Atropine does not distinguish among the M1, M2, and M3 subgroups of
muscarinic receptors.
The pupillary constrictor muscle depends on muscarinic cholinoceptor activation. This activation is
blocked by topical atropine resulting in unopposed sympathetic dilator activity and mydriasis. Atropine
also weakens the contraction of the ciliary muscle, or cycloplegia. Cycloplegia results in loss of the
ability to accommodate such that the eye cannot focus for near vision.
Pharmacodynamics
The onset of action after administration of atropine sulfate ophthalmic solution, USP 1%, is usually
within 40 minutes with maximal effect being reached in about 2 hours. The effect can last for up to 2
weeks in a normal eye.
Pharmacokinetics
The bioavailability of atropine sulfate ophthalmic solution, USP 1% was assessed in six healthy
subjects, 24 to 29 years of age. Subjects received either 0.3 mg atropine sulfate administered as bolus
intravenous injection or 0.3 mg administered as 30 μl instilled unilaterally in the cul-de-sac of the eye.
Plasma l-hyoscyamine concentrations were determined over selected intervals up to eight hours after
dose administration.
The mean bioavailability of topically applied atropine was 63.5 ± 29% (range 19 to 95%) with large
inter-individual differences. Mean maximum observed plasma concentration for the ophthalmic solution
was 288 ± 73 pg/mL. Maximum concentration was reached in 28 ± 27 min after administration. Terminal
half-life of l-hyoscamine was not affected by route of administration and was calculated to be 3 ± 1.2
hours (intravenous) and 2.5 ± 0.8 hours (topical ophthalmic).
In another placebo-controlled study, the systemic exposure to l-hyoscyamine, and the anti-cholinergic
effects of atropine were investigated in eight ocular surgery patients 56 to 66 years of age, following
single topical ocular 0.4 mg atropine dose (given as 40 microliters of atropine sulfate ophthalmic
solution, USP 1%). The mean (± standard deviation (SD)) Cmax of l-hyoscyamine in these patients was
860 ± 402 pg/mL, achieved within 8 minutes of eyedrop instillation.
Following intravenous administration, the mean (± SD) elimination half-life (t ) of atropine was
reported to be longer in pediatric subjects under 2 years (6.9 ± 3.3 hours) and in geriatric patients 65 to
75 years (10.0 ± 7.3 hours), compared to in children over 2 years (2.5 ± 1.2 hours) and in adults 16 to 58
years (3.0 ± 0.9 hours). (see Pediatric Use).
Atropine is destroyed by enzymatic hydrolysis, particularly in the liver; from 13 to 50% is excreted
unchanged in the urine. Traces are found in various secretions, including milk. The major metabolites of
atropine are noratropine, atropin-n-oxide, tropine, and tropic acid. Atropine readily crosses the
placental barrier and enters the fetal circulation, but is not found in amniotic fluid.
Atropine binds poorly (about 44%) to plasma protein, mainly to alpha-1 acid glycoprotein; age has no
effect on the serum protein binding of atropine. Atropine binding to α-1 acid glycoprotein was
concentration dependent (2 to 20 mcg/mL) and nonlinear in vitro and in vivo. There is no gender effect on
the pharmacokinetics of atropine administered by injection.
Clinical Studies
Topical administration of atropine sulfate ophthalmic solution, USP 1% results in cycloplegia and
mydriasis which has been demonstrated in several controlled clinical studies in adults and pediatric
patients. Maximal mydriasis usually occurs in about 40 minutes and maximal cycloplegia is usually
achieved in about 60 to 90 minutes after single administration. Full recovery usually occurs in
approximately one week, but may last a couple of weeks.