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 porportions 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."

Actaminophen (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. 1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid is known by the generic name of "indomethacin" for use as an antiinflammatory agent; it also has analgesic properties. However, the effect of the combination of these two drugs on the analgesic properties was not known prior to our work.

The present invention concerns an improved method of producing analgesia made possible by the discovery that a potention of analgesic or antinociceptive properties of 1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic 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 1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic acid compound, said compound 1-(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acetic 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 essay 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. responses are observed and compared with control operations.

More specifically, in an operation carried out substantially as described by Collier and co-workers, non-fasted 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 1-(p-chlorobenzoyl)-5-methoxy-2-methyl-3-acetic acid compound (hereinafter sometimes referred to as "indomethacin compound") by acetaminophen may be demonstrated. The property is illustrated employing indomethacin but 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 indomethacin 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). Indomethacin was administered as a solution in aqueous sodium hydroxide. Acetaminophen was administered as an aqueous suspension.

Thirty minutes after administration of indomethacin (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 an indomethacin 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, an indomethacin 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 0.13 to 0.27 mg/kg of body weights of indomethacin compound may be employed together with from about 3.8 to 7.5 mg/kg of body weights of acetaminophen. These amounts when expressed as doses suitable in man are in the range of from about 8 to 16 milligrams of indomethacin compound and from about 225 to 450 milligrams of acetaminophen.

Acetaminophen and an idomethacin compound may be employed in a pharmaceutical composition by intimately admixing the components 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 such as water, glycols, oils, alcohol and the like for liquid preparations; or solid carriers such as starches, sugars, kaolin, lubricants, binders, disintegrating agents and the like for solid preparations such as powders, capsules and tablets. Because of their ease of administration, tablets and capsules represent the most advantageous oral dosage unit form. The term "dosage unit form" as used 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 pharmaceutical carrier. Examples of such dosage unit forms are tablets, capsules, pills, powder packets, wafers and the like, and segregated multiples thereof. A dosage unit generally will contain from 8 to 16 milligrams of indomethacin compound and 225 to 450 milligrams of acetaminophen. The compositions thus prepared may be employed to provide the antinociceptive effects hereinbefore discussed.