Guide The Essence of Analgesia and Analgesics

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Their therapeutic pain-relieving effects were discovered serendipitously, and they were used for centuries without much rational scientific understanding of how they produced their beneficial effects. The explanation for the lack of dissociation between therapeutic and adverse effects is now understood—their major analgesic mechanism is the same as their adverse effect mechanism, namely MOR agonism, so they are in this regard in essence mono-modal or one-dimensional albeit differences in pharmacology or pharmacokinetics can result in nuances in their clinical profile.

In the absence of a molecular target, subsequent semi-synthetic and synthetic opioids were discovered by structure—activity relationships in which the dependent variable was obtained from in vivo screening using animal models such as abdominal constriction, hot plate, tail flick, etc.

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Based on the discovery of opioid receptors, the logical hypothesis was reached that there must be endogenous ligands for these receptors, and endogenous opioid peptides were soon identified. The existence of opioid receptors and endogenous ligands for these receptors revealed for the first time a cellular-level mechanistic explanation for how the exogenous opioids produced both their analgesic, and their other receptor-mediated side- effects [ 2 ].

For years, opioid pharmacology and drug discovery was directed toward compounds that had ever-greater selectivity and binding affinity for opioid receptors. There was thus a race to a more mono-modal mechanism of analgesic action. And with that came the more mono-modal opioid receptor-mediated adverse effects. As a result, with pure opioid drugs, the MOR contribution to adverse effects e. During the course of such drug discovery, however, it was inevitable that drugs with more than one pharmacologic mechanism of action were discovered [ 7 ].

For such drugs, four outcomes are possible: the mechanisms contribute either positively or negatively to the analgesic effect, and positively or to a less extent to adverse effects [ 8 ]. The latter category—drugs that have mechanisms of action that are either additive or synergistic on the analgesic endpoint, but less than additive on adverse effect endpoints—would possess clinically advantageous properties [ 9 , 10 ].

Two such drugs buprenorphine [ 11 , 12 , 13 , 14 , 15 ] and tramadol [ 16 , 17 ] were recognized after discovery. One tapentadol was designed and discovered with a specific dual mechanism as the goal [ 18 , 19 , 20 ]. Concurrent with the identification of analgesics with favorable clinical features was the discovery of pain modulatory pathways. Now collectively known as DNIC diffuse noxious inhibitory control , activity in these pathways can modify the amplitude or temporal characteristics of pain signals [ 22 ].

The monoamines, norepinephrine and serotonin, play prominent roles in the descending pathways, with variable contributions in different types of pain, at different anatomical sites, and at different periods in the progression or time course of pain. Furthermore, experience has shown that many pains involve more than one patho physiological process [ 23 , 24 ], having, for example, both a neuropathic component and a nociceptive component. Therefore, treatment of such pains with mono-mechanistic analgesics usually yields sub-optimal results either insufficient pain relief or excess adverse effects.

In such cases, better separation of therapeutic from adverse effects can be achieved by using analgesics with multiple mechanisms of action that match the multiple mechanisms of pain patho physiology [ 25 , 26 ]. Buprenorphine and tramadol are examples of analgesics that were found to have multiple mechanisms of analgesic action after their synthesis. Buprenorphine has very high affinity for MOR, which is a major mechanism of its analgesic action [ 27 ].

Tramadol produces its analgesic effect by the combined action of the enantiomers of the parent drug and enantiomers of its O -desmethyl M1 metabolite. The outcome of this approach is that tapentadol is more potent in a variety of animal models, and in clinical trials has been shown to have comparable efficacy to oxycodone, with more favorable tolerability [ 34 , 35 , 36 , 37 , 38 , 39 , 40 ].

It is a single molecule, has no analgesically active metabolite [ 41 ], and is metabolized primarily by glucuronidation rather than CYP [ 42 ]. Although tapentadol is a strong analgesic in animal models [ 43 ] and humans [ 44 , 45 , 46 ], it binds to recombinant human MOR with an affinity of 0.

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For comparison, that is about to fold lower affinity than morphine or oxycodone for MOR [ 18 , 47 , 48 ]. How does one account for a lower receptor affinity yet greater analgesic potency and efficacy of a molecule? The synergistic interaction can account for its greater functional therapeutic effect activity. Importantly, though, the two mechanisms do not interact synergistically on an adverse effect endpoint constipation [ 51 ].

There is thus a mechanistic explanation for a favorable separation between the analgesic effect and adverse effects [ 51 ]. Nevertheless, such a separation needs to be demonstrated clinically. In fact, data from clinical trials demonstrate that tapentadol produces pain relief comparable to oxycodone, but that it has a better tolerability profile, i. There is therefore compelling evidence that each component of tapentadol, opioid and non-opioid, independently contributes significantly to its analgesic effect; that is, the overall effect is a sort of hybrid of a polypharmacological ligand.

It is also suggested by the data that the contribution of the opioid component to adverse effects might be less than its contribution to analgesia. Two questions thus arise.

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To date, there has been no formal attempt to answer these questions. In the case of a drug that has enantiomers, for example, tramadol, the assessment is facilitated by the ability to administer each enantiomer separately, and observing the effect of each independently, then together.

Pain - Pathway and Analgesics (NSAIDs, Opioids, Antidepressants, and Anticonvulsants)

In the case of a single molecule such as tapentadol, the analysis is less obvious or direct. Since we are not aware of any prior attempt to answer such questions for any other drug, there is no definitive way to do so. We thus decided to take a variety of different approaches, and analyzed whether the answers would converge to an agreeably consistent answer.

Both clinical and preclinical data are taken from previously conducted studies, and no studies with human participants or animals were performed by any of the authors for this analysis. The remainder of the manuscript is designed so that the reader uninterested in the mathematical details can read the explanations and conclusions without loss of continuity. This approach proceeded as follows. Such values are obtained in a situation in which receptors are completely saturated with high concentrations of the respective compounds, and in which only one out of potentially several intracellular effector systems is considered.

That is not the case under clinical conditions. In clinical practice, the nominal equianalgesic dose of tapentadol is about 2. A similar calculation can be based on the K i values of tapentadol 0. The result of this analysis is very well in line with results from a study using naloxone and yohimbine as opioid and noradrenergic antagonists, respectively. Effect of tapentadol on CO 2 -induced stimulation of the respiratory frequency in conscious rats. Tapentadol was given i.

The inhibition of the gastrointestinal transit of charcoal is a classic animal model and a quantifiable correlate and measure of drug-induced constipation. From published results of studies using inhibition of gastrointestinal transit GIT as the endpoint in mice and rats, it is known that tapentadol inhibits GIT to a lesser extent than does morphine at equi-antinociceptive doses. It is also known that the two mechanisms of action of tapentadol interact in the third possible way on this endpoint, namely in an additive manner, in contrast to the antinociceptive synergistic and respiratory presumably counteractive endpoints.

Rotation Checklist - Acute Pain Management

For the estimation, the same antinociceptive dose as used in sections 5. From Cowan et al. The analysis set included data from randomized patients completions with end-stage joint disease day trial and from randomized patients completions with chronic low back pain or osteoarthritis pain day trial. The patients given oxycodone experienced a significantly greater number of days with either no or incomplete bowel movements over the course of the day treatment compared to placebo and compared to tapentadol.

In contrast, there was no significant difference between tapentadol and placebo in the proportions of treatment days with either no or incomplete bowel movements. Similar results were found in a randomized, double-blind, active-controlled study of patients with moderate to severe chronic malignant tumor-related pain who were given oral extended-release tapentadol or oral controlled-release oxycodone, and TEAEs treatment-emergent adverse events were recorded [ 54 ]. At equianalgesic non-inferior doses, the incidence of gastrointestinal TEAEs was lower in the tapentadol group than in the oxycodone group.

However, given the relative tight range of the values found from the estimates from preclinical studies, an estimation based on the clinical trial data was deemed warranted, if sufficiently circumspect in its conclusions. In the absence of data on tapentadol brain levels in humans, the estimate is necessarily based on the effect domain. In the cLBP study [ 56 ], the odds of experiencing constipation were significantly lower with tapentadol than with oxycodone.

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In the OA pain studies, there were substantially lower incidences of gastrointestinal-related TEAEs associated with treatment with tapentadol than with oxycodone [ 52 ], and tapentadol was better tolerated than oxycodone, largely due to fewer gastrointestinal side-effects [ 57 ]. OA pain Afilalo et al. OA pain Serrie et al. Pooled analysis Lange et al. A study that has recently been published by van der Schrier et al.

It is the first such study that has directly compared the respiratory effects of tapentadol and oxycodone. Significantly greater respiratory depression was observed following exposure to oxycodone than to tapentadol. The data indicate a respiratory inhibition potency ratio of 7. This ratio exceeds the commonly used ratio for analgesia of 5.

However, since the above previous approaches were rigorous quantitative methods and gave similar results, it seems that the data from this study should not be wasted, and should be assessed in some way, even if only a qualitative approach is all that can be used. Although the effect difference was not quantitatively described in the publication, tapentadol produced approximately half of the respiratory depression produced by an equianalgesic dose of oxycodone Fig.

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This is consistent with available data on the in vivo activity of tapentadol. Thus, when considering the balance between analgesia and opioid-typical adverse effects, the opioids are rather mono-mechanistic. The question is: to what extent does the opioid component contribute to analgesia on the one hand, and to adverse effects on the other hand? Estimates using clinical trial data yield results similar to the estimates using in vitro and in vivo animal data. The results of this analysis are consistent with, and help explain, the favorable clinical characteristics of tapentadol with regards to opioid-induced side effects.

The results of our analysis also suggest that for a drug to be a strong analgesic, it does not have to be a strong opioid, and that this distinction is particularly important when considering the side effects of strong analgesics.


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