Analgesic Potentiation

This invention is directed to improved methods and compositions for producing analgesia.

One of the long existing primary goals of medicine is the relief of pain. Relief is sought most generally by the administration of analgesic drugs which produce a state of decreased awareness of the sensation and increase of the pain threshold.

Almost all potent analgesics evoke reactions other than the relief of pain. Some of the reactions are gastrointestinal disturbances, nausea, constipation and vomiting. Among the more serious of the side reactions and one frequently found in analgesic drugs is respiratory depression. Thus, in the use of analgesics in man, considerations other than the primary effect (analgesia), must be made and drugs for pain relief are sought which have maximum analgesic effect accompanied by minimum side reactions. It is difficult to satisfy these requirements with a single chemical entity since generally a potent analgesic has accompanying serious side reactions while a drug with little or no side effects are generally less effective as an analgesic.

Thus, there is a continuing search for a combination of two or more drugs whereby the total quantity of drug can be reduced and which can be employed in such proportions as to produce maximum analgesic effect with little or no side effects. When one or both of the components of a combination is known to possess pain relieving properties but these properties are increased many fold, the net effect of the combination is commonly referred to as "potentiation."

Acetaminophen (p-acetaminophenol) is recognized as an analgesic agent with useful and safe antinociceptive properties. However, in certain instances, high doses must be employed to effectively reduce pain. 6-Methoxy-.alpha.-methyl-2-naphthaleneacetic acid is known by the generic name of "naproxen" for use as anti-inflammatory, analgesic or antipyretic agent. However, the effect of the combination of these two drugs on the analgesic properties has not been known prior to our work.

The present invention concerns an improved method of producing analgesia made possible by the discovery that a potentiation of analgesic or antinociceptive properties of 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid or its salt is produced by administration with acetaminophen in specific proportions. The use of the combination in the suppression of pain is unexpectedly much greater than that which would result from simply the additive effect of the components.

The novel and unexpected superior analgesic properties may be achieved by the simultaneous or sequential oral administration of (1) a 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid compound, said compound being 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid represented by the formula ##STR1## or a therapeutically acceptable salt thereof and (2) acetaminophen. The therapeutically acceptable salts are those obtained from appropriate organic or inorganic bases. Preferred salts include these of sodium potassium and the like.

The efficacy of the novel combination in producing antinociceptive properties is particularly seen in acetylcholine bromide abdominal constriction assay of Collier et al [Brit. J. Pharmacol. Chemotherap. 32, 295-310 (1968)] sometimes referred to as the "mouse writhing test." In the test, mice are dosed with test drug combinations and thereafter injected interperitoneally with acetylcholine bromide and the abdominal constriction responses or block of abdominal constriction responses are observed and compared with control operations.

More specifically, in an operation carried out substantially as described by Collier and co-workers, nonfasted albino mice weighing 18-24 grams were dosed with combinations of one test drug in selected fixed doses together with a second test drug at variable doses for each fixed dose. These were compared with animals dosed with (a) the first test drug at the same doses used in combination but employing saline instead of the second drug, (b) the second test drug at the doses used in combination but employing saline instead of the first drug, and (c) a saline control containing no drug. Each of the drugs, when separately employed, were found to be inactive in the acetylcholine bromide abdominal constriction test at the test dose level.

In carrying out the tests, the test compositions and the control compositions both with and without drugs were administered orally to the test mice. After about 30 minutes the mice were injected intraperitoneally with acetylcholine bromide and the abdominal constriction responses compared (corrected for saline responses as necessary). The results are expressed as percent block.

When the percent block of abdominal constriction observed with the fixed dose was 5 percent or more, the percent block observed with each of the combination dosage levels was corrected with respect to the percent block observed with a fixed dose drug using Abbott's formula for natural mortality.

Likewise, when the response to acetylcholine bromide in the saline control group was 95 percent or less, the response observed with each of the variable dosage levels used in the ED.sub.50 determination was similarly corrected using Abbott's formula for natural mortality. ED.sub.50 and 95 percent confidence limits were calculated according to Finney's probit analysis procedure. (Finney, D. J. 1964, Probit Analysis, Second Edition, University Press, Cambridge.) All ED.sub.50 for a given comparison were calculated using a common slope following a test for parallelism. All paired dose response curves were found to be parallel.

Employing the above-described procedures, potentiation of the analgesic properties of a 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid compound (hereinafter sometimes referred to as "naproxen compound") by acetaminophen may be demonstrated. The property is illustrated employing naproxen (acid form) but it is to be understood that it is not limited thereto.

Non fasted male albino mice weighing 18-24 grams were dosed initially per os (p.o.) with naproxen at various doses and with saline as seen in Tables IA and IB and fifteen minutes later with acetaminophen at 25 mg/kg or at 50 mg/kg. All dose levels of test drug and saline control were given as and are expressed as per os. Twenty mice were employed for each dosage level. As controls, a similar number of mice were dosed with (a) acetaminophen (25 mg/kg) plus saline (10 ml/kg) or acetaminophen (50 mg/kg) plus saline (10 ml/kg) and (b) double dose of saline (10 ml/kg). Naproxen was administered as an aqueous suspension. Acetaminophen was administered as an aqueous suspension.

Thirty minutes after administration of naproxen (and 15 minutes after administration of acetaminophen), the mice were injected intraperitoneally (i.p.) with 5.5 mg/kg of acetylcholine bromide and observed for the presence or absence of the abdominal constriction response. ED.sub.50 values were determined from the observed values applying where pertinent corrections previously discussed. The results are seen in Tables IA and IB.

(Where "mg/kg" or "ml/kg" is employed, "kg" is in reference to body weight.)

The foregoing results illustrate the potentiation of the analgesic or antinociceptive properties of a naproxen compound by acetaminophen.

The process of the present invention, namely, a method of producing analgesia, comprises orally administering to subjects, i.e., human and other warm-blooded animals suffering from pain, a naproxen compound and acetaminophen in amounts sufficient to have an antinociceptive effect. The agents may be administered simultaneously or sequentially and either agents may be administered first. By the administration of the amounts of the agents as hereinafter set forth, an antinociceptive interaction between the drugs is achieved which is wholly unexpected from the known properties of the components. The active agents may be administered with or without carrier. One method of administration is by use of compositions in unit dosage form which provides a convenient simultaneous administration method.

From the foregoing test results on mice and the known dosage ranges of the components as applied to man when employed alone, it is determined that generally from about 1.4 to 2.1 mg/kg of naproxen compound may be employed together with from about 2.8 to 7.5 mg/kg of acetaminophen. These amounts when expressed as doses suitable for man are in the range of from about 85 to 125 milligrams of naproxen compound and from about 170 to 450 milligrams of acetaminophen.

The outstanding properties may be effectively utilized by use of the novel pharmaceutical compositions of the present invention. To prepare the pharmaceutical compositions of this invention, a 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid compound and acetaminophen as a potentiating agent, are intimately admixed with a pharmaceutically acceptable carrier suitable for oral administration. In preparing the compositions in oral dosage form, any of the usual pharmaceutical media may be employed, including liquid carriers such as water, glycols, oils, alcohols and the like for oral liquid preparations such as suspensions, elixirs and solutions; and solid carriers such as starches, sugars, kaolin, calcium stearate, ethyl cellulose, etc. including materials which function as lubricants, binders, disintegrating agents and the like for powders, capsules and tablets. Because of their ease in administration, tablets and capsules represent the most advantageous oral dosage form, these compositions employ solid pharmaceutical carriers such as the aforementioned starches, sugars, kaolin and the like, generally with a lubricant such as calcium stearate. It is especially advantageous to formulate the aforementioned pharmaceutical compositions in dosage unit form for ease of administration and uniformity of dosage. The term "dosage unit form" as used in the specification and claims herein refers to physically discrete units suitable as unitary dosages, each unit containing a predetermined quantity of active ingredient calculated to produce the desired therapeutic effect in association with the required pharmaceutical carrier. Examples of such dosage unit forms are tablets, capsules, pills, powder packets, wafers, teaspoonfuls, tablespoonfuls and the like, and segregated multiples thereof. A dosage unit generally will contain from about 85 to 125 milligrams of a 6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid compound as primary active ingredient together with from about 170 to 450 milligrams of acetaminophen.

The following examples are given to illustrate the novel compositions and are not to be construed as limiting the invention in spirit or in scope.

1000 hard gelatin capsules, each containing 105 milligrams of (+)-6-methoxy-.alpha.-methyl-2-naphthaleneacetic acid (naproxen) as primary active ingredient and 310 milligrams of acetaminophen as potentiating agent are prepared from the following formulation:

A uniform mixture of the ingredients is prepared by blending, and used to fill two-piece hard gelatin capsules. The capsules are suitable to be used for providing satisfactory analgesic effect upon administration to subjects suffering from pain.

Gelatin capsules are prepared as described in Example I except that in the formulation, 115 grams of sodium 6-methoxy-.alpha.-methyl-2-naphthaleneacetate (naproxen sodium) is employed as the primary active ingredient and 350 grams of acetaminophen is employed as the potentiating agent, thus providing capsules each containing 115 milligrams of naproxen sodium and 350 milligrams of acetaminophen.

1000 compressed tablets, each containing as the primary active ingredient 115 milligrams of naproxen and 350 milligrams of acetaminophen as potentiating agent are prepared from the following formulation:

The finely powdered ingredients are mixed well and granulated with 10 percent starch paste. The granulation is dried and compressed into tablets .