Peripheral antialgesics: a review

Efficacy and safety of ketorolac tromethamine 0.5% and levocabastine 0.05%: a multicenter comparison in patients with seasonal allergic conjunctivitis

This multicenter, double-masked, randomized, parallel-group study compared the efficacy and safety of ketorolac tromethamine 0.5% ophthalmic solution with levocabastine 0.05% and ketorolac tromethamine vehicle in patients with seasonal allergic conjunctivitis. One drop of ketorolac, levocabastine, or vehicle was instilled in each eye four times daily for 6 weeks. In the majority of efficacy variables, ketorolac produced the greatest improvements, followed by levocabastine and vehicle. Ketorolac was significantly more effective (P < .05) than vehicle in reducing mean itching scores, palpebral hyperemia, bulbar hyperemia, and edema. Patients treated with ketorolac reported significant improvements (P < .05) in their ability to sleep and to concentrate on work, compared with those who received vehicle. No significant differences were noted among the treatment groups in safety or tolerability. Ketorolac tromethamine 0.5% ophthalmic solution instilled four times daily is effective and safe in reducing the signs and symptoms of seasonal allergic conjunctivitis.

Efficacy and safety of intracavernosal alprostadil in men with erectile dysfunction. The Alprostadil Study Group

BACKGROUND

Erectile dysfunction is a common medical problem affecting many men. Although several intracavernosal therapies are available, their efficacy and safety have not been studied systematically.

METHODS

We investigated the efficacy and safety of alprostadil formulated for intracavernosal treatment in three separate multi-institutional, prospective studies in men with erectile dysfunction of vasculogenic, neurogenic, psychogenic, and mixed causes. Clinical and laboratory evaluations of erection and the feasibility of satisfactoriness of sexual activity as assessed both by the men and by their partners were the primary measures of efficacy.

RESULTS

In a dose-response study of 296 men, all doses of alprostadil were superior to placebo and there was a significant dose-response relation (P < / = 0.001), resulting in higher response rates with increasing doses of alprostadil (from 2.5 to 20 microg). In a dose-finding study of 201 men, the minimal effective dose was < / = 2 microg in 23, 20, 38 and 23 percent of men with erectile dysfunction of neurogenic, vasculogenic, psychogenic, or mixed causes, respectively. In a six-month self-injection study in 683 men, the participants reported being able to have sexual activity after 94 percent of the injections. The men and their partners rated the sexual activity as satisfactory after 87 and 86 percent of the injections, respectively. Penile pain, usually mild, occurred in 50 percent of the men at some time but after only 11 percent of the injections. Prolonged erections occurred in 5 percent of the men, priapism in 1 percent, penile fibrotic complications in 2 percent, and hematoma or ecchymosis in 8 percent.

CONCLUSIONS

In men with erectile dysfunction, intracavernosal injection of alprostadil is an effective therapy with tolerable side effects.

Central analgesic effects of aspirin-like drugs

Aspirin-like drugs mainly include paracetamol, salicylates and other non-steroidal anti-inflammatory drugs, and metamizole. Their analgesic effect is classically ascribed to a peripheral site of action, within the pain-processing site. There is, however, convincing evidence that a central component contributes to the overall analgesia provided by these agents. Experimental and clinical studies referring to this challenging proposal are reviewed here. The exact site and mode of action of aspirin-like drugs within the central nervous system remains controversial. It is likely that supraspinal mechanisms play an important role. Some experiments lend support to the involvement of monoaminergic control systems. Other data indicate that these drugs act centrally through the inhibition of cyclo-oxygenase activity. The interactions between prostaglandins and various neurotransmitters suggest that both mechanisms may be linked.

Non-steroidal anti-inflammatory drugs and spinal nociceptive processing

Non-steroidal anti-inflammatory drugs have a direct action on spinal nociceptive processing in vivo with a relative order of potency which correlates with their capacity as inhibitors of cyclooxygenase activity. However, recent clinical surveys and new in vivo evidence strongly suggest that for some of these agents, centrally mediated analgesia may also be achieved by additional mechanisms, which are independent of prostaglandin synthesis inhibition. In this review we explore the likelihood for such mechanisms following an extensive survey of existing data. The implications of these mechanisms are discussed in the light of our current understanding of spinal nociceptive processing.

The spinal actions of nonsteroidal anti-inflammatory drugs and the dissociation between their anti-inflammatory and analgesic effects

The traditional classification of nonsteroidal anti-inflammatory drugs (NSAIDs) as exclusively 'peripherally acting' agents is no longer valid. For many of these agents there is a growing body of evidence in favour of an additional central mechanism for their anti-inflammatory and analgesic effects. This view is further supported by the recent discovery that a substantial component of the hyperalgesia and allodynia that characterise postinjury hypersensitivity occurs in the CNS, notably the spinal dorsal horn. An important corollary is that inhibition of central nociceptive processing may represent an important analgesic mode of action for those NSAIDs that are effective in the management of pain after tissue injury. Historically, attempts to group this heterogeneous class of compounds into a single entity are largely derived from the observation that the majority of clinically useful NSAIDs are weak organic acids (pKa 3 to 5), bind extensively to plasma albumin (= 99%), and inhibit (to varying degrees) prostaglandin synthesis. However, the significance of these various unifying features is becoming increasingly obscure. While inhibition of prostaglandin synthesis apparently remains an important analgesic mode of action for NSAIDs both in the periphery and the CNS, other mechanisms should be considered. Some NSAIDs, in addition to their effects on prostaglandin synthesis, also affect the synthesis and activity of other neuroactive substances believed to have key roles in processing nociceptive input within the dorsal horn. It has been argued that these other actions, in conjunction with inhibition of prostaglandin synthesis, may synergistically augment the effects of NSAIDs on spinal nociceptive processing. Despite much effort, it remains a formidable task to assess the significance of these differential mechanisms upon clinical pain states. In the meantime, however, it may be possible, on the basis of in vivo studies, to evaluate the impact of putative spinal analgesic mechanisms that are unrelated to inhibition of prostaglandin synthesis. This approach has recently been extended to include the identification of pharmacokinetic and clinical correlates of these derived in vivo parameters, and in this way attempt to demonstrate clinical relevance.

Central analgesic activity of nonsteroidal antiinflammatory drugs in animal and human pain models

Nonsteroidal antiinflammatory drugs (NSAIDs) often are considered peripherally acting analgesics. This traditional view has been challenged by several authors who have put forward evidence for a central analgesic action for both acidic and nonacidic NSAIDs. The most compelling evidence, reviewed in this report, comes from well-defined animal pain models in which central administration of low doses of NSAIDs provides effective analgesia. In humans, the nociceptive flexion reflex provides a valuable means of dissociating central and peripheral effects. The mechanism of the central action of NSAIDs remains unclear. Several lines of evidence implicate the inhibition of central prostaglandin synthesis, particularly in models with a strong hyperalgesic component to their pathophysiology. However, not all potent inhibitors of prostaglandin synthesis are active by central routes of administration, and it seems likely that other actions may be involved.

Clinical evaluation of ketorolac tromethamine 0.5% ophthalmic solution for the treatment of seasonal allergic conjunctivitis

We evaluated 148 patients with allergic conjunctivitis in a double-masked, paired comparison clinical trial comparing ketorolac 0.5% ophthalmic solution with vehicle. Patients received one drop of each study medication in preassigned eyes, four times a day, for seven days. Both treatments showed significant changes from baseline in the signs and symptoms associated with allergic conjunctivitis. Evaluations at the final visit (day 7 or 8) showed that ketorolac-treated eyes had a significant treatment response when compared to vehicle-treated eyes for conjunctival inflammation (p = 0.010), ocular itching (p = 0.006), swollen eyes (p = 0.002), discharge/tearing (p = 0.021), foreign body sensation (p = 0.035), and conjunctival injection (p = 0.016). Mean scores evaluating the overall therapeutic effect of the study treatments at the completion of the study were higher for ketorolac-treated eyes than for vehicle-treated eyes as rated by investigators (p = 0.004) and study patients (p < 0.001). Results of this study confirmed the trends of a previous study showing that ketorolac 0.5% ophthalmic solution applied topically is an effective therapy for allergic conjunctivitis.

Where are peripheral analgesics acting?

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Acceptability and efficacy of two associations of paracetamol with a central analgesic (dextropropoxyphene or codeine): comparison in osteoarthritis

A double-blind randomized parallel group trial was undertaken to compare the acceptability and efficacy of 2 forms of analgesic treatment, DI-Antalvic (Houde Laboratories, Puteaux, France) (30 mg dextropropoxyphene and 400 mg paracetamol per capsule) and Efferalgan-Codeine (UPSA Laboratories, Rueil Malmaison, France) (30 mg codeine and 500 mg paracetamol per tablet) prescribed for 1 week at doses of 6 capsules/day and 6 tablets/day, respectively, in 141 outpatients with active osteoarthritis of the knee or hip. The principal aim of the trial was concerned with acceptability, with efficacy as its secondary aim. The principal trial criterion was defined as overall assessment of acceptability by the patient at the end of the trial (success or failure) or by treatment dropouts because of an adverse effect (failure). Comparability of the groups was confirmed before any treatment regarding the physical characteristics of the patients, characteristics of osteoarthritis, and the initial level of pain and functional consequences of pain. Results show that the analgesic efficacy of the treatment was similar, but that the acceptability of Efferalgan-Codeine was significantly worse than that of DI-Antalvic: 53% failure with Efferalgan-Codeine versus 29% failure with DI-Antalvic (P = .005). Other trials of the same type would seem necessary (comparison of lower doses, other types of pain) before being able to generally extrapolate such findings.

The effect of topical administration of indomethacin on symptoms in corneal scars and edema

We conducted a masked randomized study of 50 patients to evaluate the effect of administration of topical indomethacin 1% suspension on symptoms in corneal scars, edema, infiltrates, and erosions. Patients with symptoms (photophobia, pain, itching, burning sensation, foreign-body sensation, and tearing) were treated with topically administered indomethacin 1% or placebo and monitored for eight weeks. The severity of the complaints was rated and the scores were evaluated (Wilcoxon rank-sum test). Of the 25 patients treated with indomethacin, 21 (84%) had improvement in symptoms and the severity of each of the symptoms was significantly decreased. Of the 25 patients treated with placebo, one (4%) had improvement in symptoms without statistical change of the severity of the symptoms. However, when the placebo-treated patients received indomethacin drops, the symptoms were significantly decreased (P less than .002). This study suggests that topical administration of indomethacin 1% may reduce ocular symptoms in patients with corneal scars, edema, or erosions.

Double-blind evaluation of analgesic efficacy of orally administered diclofenac, nefopam, and acetylsalicylic acid (ASA) plus codeine in chronic cancer pain

The analgesic efficacy and toxicity of oral diclofenac sodium 50 mg (q.i.d.) vs. nefopam 60 mg (q.i.d.) and a combination of 640 mg ASA and 40 mg codeine (q.i.d.) in cancer patients with moderate to severe chronic pain has been evaluated in a randomized double-blind study. Planned duration of treatment was 10 days. Pain intensity was evaluated by a visual analog scale. The length of patient participation in the trial, the patient's final global evaluation and the incidence of side effects were also evaluated. Ninety-nine patients were enrolled in the study. All treatments produced a statistically significant pain relief (P less than 0.01) without differences among groups but only 26 of 99 patients (26.3%) completed the planned treatment period. Mean time in the study was 4.65 days. Inefficacy and side effects were the main reasons for premature treatment interruption. Patients treated with nefopam had a significantly shorter period in the study than patients treated with the other 2 treatments. Adverse effects were slightly more frequent with the nefopam and ASA + codeine regimens. The 3 therapeutic regimens appear to be similar as to analgesic efficacy, but diclofenac presents the advantage of a slightly better safety profile than nefopam and the ASA + codeine combination.

The formalin test in mice: dissociation between inflammatory and non-inflammatory pain

The formalin test in mice is a valid and reliable model of nociception and is sensitive for various classes of analgesic drugs. The noxious stimulus is an injection of dilute formalin (1% in saline) under the skin of the dorsal surface of the right hindpaw. The response is the amount of time the animals spend licking the injected paw. Two distinct periods of high licking activity can be identified, an early phase lasting the first 5 min and a late phase lasting from 20 to 30 min after the injection of formalin. In order to elucidate the involvement of inflammatory processes in the two phases, we tested different classes of drugs in the two phases independently. Morphine, codeine, nefopam, and orphenadrine, as examples of centrally acting analgesics, were antinociceptive in both phases. In contrast, the non-steroid anti-inflammatory drugs indomethacin and naproxen and the steroids dexamethasone and hydrocortisone inhibited only the late phase, while acetylsalicylic acid (ASA) and paracetamol were antinociceptive in both phases. The results demonstrate that the two phases in the formalin test may have different nociceptive mechanisms. It is suggested that the early phase is due to a direct effect on nociceptors and that prostaglandins do not play an important role during this phase. The late phase seems to be an inflammatory response with inflammatory pain that can be inhibited by anti-inflammatory drugs. ASA and paracetamol seem to have actions independent of their inhibition of prostaglandin synthesis and they also have effects on non-inflammatory pain.

The antialgesic drugs: human therapeutic correlates of their potency in laboratory animal models of hyperalgesia

This survey discusses the correlation between the oral potency of antialgesic drugs in several pharmacology laboratories and their human oral dose in clinical practice. We also present a brief overview of a few biological assays that have been successfully used to direct the synthesis of newer antialgesic drugs. The laboratory assay that our analysis showed to be most predictive of the clinical analgesic dose is based upon the response of rats to flexion of an arthritic joint. Laboratory ED50 values from the ACh-induced abdominal constriction assay in mice are nearly as predictive while the predictive power of the yeast-induced hyperalgesia assay in rats is somewhat less. Probably because of the small number of experiments, the correlation between the efficacy of these agents in a canine model of synovitis and their clinical doses only reached borderline statistical significance (p = 0.0651). Regression equations are presented that permit calculations of single clinical analgesic doses from efficacy data in individual tests. Calculation of stepwise multiple regression showed that the clinical dose could be best predicted when efficacy data obtained in the joint flexion assay in rats and the ACh-induced constriction assay in mice are both taken into account. We have concluded that the effective doses are highly predictive of clinical efficacy because these animal assays have been designed to reflect the action of drugs upon prostanoid-induced hyperalgesia.

Comparison of the analgesic effect of a corticosteroid and paracetamol in patients with pain after oral surgery

In a double-blind crossover study identical oral surgical procedures were performed on two separate occasions in 24 patients. After one operation they were given capsules of methylprednisolone. A total of 84 mg was administered in decreasing doses for 4 days, starting with 24 mg 2 h after surgery. After the other operation 1 g paracetamol was given at 2, 6 and 9 h after surgery, then 500 mg four times daily for the next 2 days. Several measurements/assessments were recorded for paired comparisons of the postoperative courses. There was no significant difference in pain scores when comparing the two treatments. However, 3 and 4 h after the operation, there was a tendency towards better pain relief with paracetamol (P greater than 0.05). The results suggest that methylprednisolone, after a delay, may exert analgesic efficacy comparable to a standard regimen with paracetamol. On the third postoperative day methylprednisolone reduced measured swelling by 29% compared to paracetamol (P = 0.03). Overall preference scores evaluated the two treatments as equally efficient on the day of surgery. The first and second postoperative day the scores were in favour of methylprednisolone (P = 0.03). Present and previous results in this model suggest that corticosteroids deserve attention as a means of reducing both pain and inflammation after surgery and trauma.

Mode of analgesic action of dipyrone: direct antagonism of inflammatory hyperalgesia

Dipyrone blocked carrageenin-induced oedema and hyperalgesia in a dose-dependent manner. In contrast with indomethacin, paracetamol and acetyl salicylic acid, much lower doses of dipyrone were necessary for blocking hyperalgesia (ED50 = 19 mg/kg, i.p.) than oedema (180 mg/kg, i.p.) Dipyrone administered intraperitonially or intraplantarly was able to antagonise PGE2-, isoprenaline- and calcium chloride-induced hyperalgesia, effects which are not observed with non-steroid anti-inflammatory drugs. Systemic or local administration of dipyrone had no effect upon Db-cAMP-induced hyperalgesia while a centrally acting analgesic, morphine, given systemically, was highly effective. These results support our suggestion that the mechanism of action of dipyrone is different from that of classical non-steroidal anti-inflammatory drugs. Although the site of action is peripheral its analgesic effect does not derive from inhibition of the synthesis of prostaglandins but is exerted via direct blockade of the inflammatory hyperalgesia.