CLINICAL PHARMACOLOGY
Mechanism of Action
The mechanism of action of diclofenac is similar to that of other nonsteroidal anti-inflammatory drugs. Diclofenac inhibits the enzyme, cyclooxygenase (COX), an early component of the arachidonic acid cascade, resulting in the reduced formation of prostaglandins, thromboxanes and prostacylin. It is not completely understood how reduced synthesis of these compounds results in therapeutic efficacy.
Pharmacodynamics
Diclofenac, the active component of Voltaren® Gel has anti-inflammatory, anti-nociception, and antipyretic effects.
Pharmacokinetics
The pharmacokinetics of Voltaren® Gel were assessed in healthy volunteers following repeated applications during 7 days of Voltaren® Gel to 1 knee (4 x 4 g per day) or to 2 knees and 2 hands (4 x 12 g per day) versus the recommended oral dose of diclofenac sodium for the treatment of osteoarthritis (3 x 50 mg per day). A summary of the pharmacokinetic parameters is presented in Table 2.
Table 2. Pharmacokinetic Parameters and Comparison of Voltaren® Gel to Oral Diclofenac Sodium Tablets After Repeated Administration
|
Treatment
|
C
max
(ng/mL)
Mean ± SD
% of Oral (CI)
|
t
max
(hr)
Median (range)
|
AUC
0-24
(ng•h/mL)
Mean ± SD
% of Oral (CI)
|
Voltaren® Gel
4 x 4 g per day
(=160 mg diclofenac sodium per day) |
15± 7.3
0.6% (0.5-0.7) |
14 (0-24) |
233 ± 128
5.8%
(5-6.7) |
Voltaren® Gel
4 x 12 g per day
(=480 mg diclofenac sodium per day) |
53.8 ± 32
2.2%
(1.9-2.6) |
10 (0-24) |
807 ± 478
19.7%
(17-22.8) |
Diclofenac sodium tablets, orally
3 x 50 mg per day
(=150 mg diclofenac sodium per day) |
2270 ± 778
100% |
6.5 (1-14) |
3890 ± 1710
100% |
Cmax = maximum plasma concentration; tmax = time of Cmax; AUC0-24 = area
under the concentration-time curve; SD = standard deviation; CI = confidence interval.
Systemic exposure (area under the concentration-time curve) and maximum plasma concentrations of diclofenac are significantly lower with Voltaren® Gel than with comparable oral treatment of diclofenac sodium.
Systemic exposure with recommended use of Voltaren® Gel (4 x 4 g per day applied to 1 knee) is on average 17 times lower than with oral treatment. (Basis: treatment with Voltaren® Gel of 1 knee, 4 times a day versus 50 mg, 3 times a day of oral diclofenac tablets). The amount of diclofenac sodium that is systemically absorbed from Voltaren® Gel is on average 6% of the systemic exposure from an oral form of diclofenac sodium.
The average peak plasma concentration with recommended use of Voltaren® Gel (4 x 4 g per day applied to 1 knee) is 158 times lower than with the oral treatment.
The pharmacokinetics of Voltaren® Gel has been tested under conditions of moderate heat (application of a heat patch for 15 minutes prior to gel application) and of moderate exercise (first gel application followed by a 20-minute treadmill exercise). No clinically relevant differences of systemic absorption and of tolerability were found between applications of Voltaren® Gel (4 x 4 g per day on 1 knee) with and under the conditions tested. However, the pharmacokinetics of Voltaren® Gel were not tested under the condition of heat application following gel application. Therefore, concurrent use of Voltaren® Gel and heat is not recommended.
NONCLINICAL TOXICOLOGY
Carcinogenesis, Mutagenesis, Impairment of Fertility
Carcinogenicity studies in mice and rats administered diclofenac sodium as a dietary constituent for 2 years at does up to 2 mg/kg/day resulted in no significant increases in tumor incidence corresponding to a human equivalent dose approximately 0.5- and 1-fold (mouse and rat, respectively) of the maximum human topical dose of Voltaren® Gel (based on bioavailability and body surface area comparison).
In a dermal carcinogenicity study conducted in albino mice, daily topical applications of a diclofenac sodium gel product for two years at concentrations up to 0.035% diclofenac sodium (a 29-fold lower diclofenac sodium concentration than present in Voltaren® Gel) did not increase neoplasm incidence.
In a photococarcinogenicity study conducted in hairless mice, topical application of a diclofenac sodium gel product at doses up to 0.035% diclofenac sodium (a 29-fold lower diclofenac sodium concentration than present in Voltaren® Gel) resulted in an earlier median time of onset of tumors.
Diclofenac was not mutagenic or clastogenic in a battery of genotoxicity tests that included the bacterial reverse mutation assay, in vitro mouse lymphoma point mutation assay, chromosomal aberration studies in Chinese hamster ovarian cells in vitro, and in vivo rat chromosomal aberration assay of bone marrow cells.
Diclofenac did not affect male or female fertility in rats at doses up to 4 mg/kg/day which induced toxicity, corresponding to a human equivalent dose approximately 2-fold greater than the maximum human topical dose of Voltaren® Gel (based on bioavailability and body surface area comparison).
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