*Corresponding author
Fisiopatología i Tratamiento del Dolor –Fundación IMIM
Parc de Recerca Biomédica de Barcelona (PRBB)
In the daily clinical practice, combined pharmacotherapy is often used when a single drug (monotherapy) treatment is not enough to relief moderate-severe pain in patients. The use of experimental animal models offers us great tools for the translation of data from animals to human. However, the accurate design of the preclinical study is a requisite to obtain valid outcomes. Using the isobolographic analysis we can easily establish differences between synergistic or additive responses expected from drug combinations, improving the management and treatment of pain.
Keywords: Animal models; Analgesic Effects; Drug combination; Isobolographic Analysis.
Analgesic drug combinations have demonstrated to be beneficial in the management of acute and chronic pain in humans1-2. Combination pharmacotherapy is regularly used when single drug (monotherapy) treatment is not enough to relief moderate-severe pain in patients. Generally, the combined drugs are analgesics with different mechanisms of action,3-7 although antidepressants, anticonvulsivants or anti-inflammatory can be also associated in order to improve the analgesic efficacy and/or to prolong analgesic duration, decreasing the doses of the individual drugs and therefore, minimizing adverse effects. However, in clinical practice, these mixtures are frequently utilized empirically, not knowing if the drugs administrated are able to interact, or not, among themselves. Consequently, we could suggest that the benefits of two or more drugs simultaneously administered should be evaluated before the combination can be considered clinically useful.
The objective of this brief article is to express a personal opinion about this topic, clarifying adequate experimental methodology to reach a better knowledge and management of pain.
Ethical aspects limit the investigation of this topic in humans. Thus, the pharmacological effects (beneficial and/or adverse) attained with combination treatment can be studied in different animal models; although the use of conscious animals in pain research is controversial by ethical and technical difficulties.8-11In my opinion, Mogil (2009), in his review titled “Animal models of pain: progress and challenges” describes exactly the complexity of behavioral animal models on pain studies, and sometimes the lack of translational results obtained from preclinical studies. In general, animal models are important for increasing the knowledge of the mechanisms of pain and development of effective therapy for its optimal management. Nowadays, animal welfare regulation progresses in the standards of housing, manipulation, animals’ health and behaviour, improving the precision and diminishing the variability by the experimental model. Thus, depending on the subject (animal species, strain (mutant, transgenic), sex, age), aetiology of pain (nociceptive, inflammatory, neuropathic, oncologic, or postsurgical), the behavior response observed after applying noxious stimuli, and easily scored (withdrawal, jumping, writhes, licking, shaking or biting, among others), basic pain researchers can use a battery of models which try to reproduce/mimic the expected progression of the pathological situation observed in patients.
Methodologically, it is easy. First, dose-response curves for each individual drug, and the experimental test (thermal, mechanical, chemical) evaluated are calculated, using at least four doses per curve. From the corresponding dose-response curves, and by least-square linear regression analysis, the dose that produces, for example, a 50% or 80% effect (ED50-80) of each drug, is calculated; an additional dose-response curve is generated with each equieffective dose (ED20-50-80 or others levels of effect) and combined at fixed ratio (i.e., 1:1, 3:1 or 1:3). The ED50 obtained experimentally is statistically compared with the theoretic ED50 obtained on the basis of the assumption that the effects of the drugs are additive (no interaction exists).
For the graphic representation of the results, the isobologram is a common method for assessing the presence (or not) of interaction when two (or more) drugs are administered in combination. The isobologram, for a given level of response or effect (i.e., ED50-45 or 60), is constructed, where equipotent doses are plotted on the x- and y-axes the ED values of each drug alone, obtained from their respective dose–response curves. The line that connects these doses corresponds to the theoretical additive line (isobole). Then, the doses of the combination are also plotted. If the experimental point falls below or above the isobole, synergy or antagonism, respectively, may be present. The point represented in the isobole line is the theoretical additive point, and the point obtained after the administration of the combination is the experimental point; theoretical and experimental points are compared using a Student t test. Moreover, the interaction index (I.I.) describes the magnitude of the effect of the combination. If the value is close to 1, the interaction is additive. Values lower than 1 are an indication of synergistic interaction, while values higher than 1 correspond to a sub-additive or antagonistic interaction.12-14 Using this type of analysis, an interaction can be established only for a given drug ratio and level of effect, and the results cannot be extrapolated to other drug combinations.
The results obtained from isobolographic analysis in animal models can show that not only the mechanism of action of the analgesic combined drugs determine the presence of synergism or antagonism between them, but also other factors as the dose ratio used,15 the nociceptive test evaluated,16 or the presence or not of opioid tolerance,17 suggesting that the combination evaluated could be adequate (or not) in clinical trials in humans.
Physicians observe the problems that arise in their daily practice and move them to the lab where the basic scientists, try to find an explanation. The use of experimental animal models offers us great tools for the translation of data from animals to human. However, the accurate design of the preclinical study is a requisite to obtain valid outcomes, taking in mind the potential benefits possibly obtained from the experiments. So, physicians and basic scientists must establish a positive interaction between them to improve the management and treatment of pain.