Proc. West. Pharmacol. Soc. 50: 75 -77 (2007) Palmitic Acid Analogues Exhibiting Antinociceptive Activity in Mice Myrna Déciga-Campos 1,2* , Rosa Mariana Montiel-Ruiz 3 , Gabriel Navarrete-Vázquez 3 and Francisco Javier López-Muñoz 4 1 Sección de Estudios de Posgrado e Investigación, Escuela Superior de Medicina del Instituto Politécnico Nacional, México, 2 Unidad de Investigación, Instituto Nacional de Enfermedades Respiratorias Secretaria de Salud, México, 3 Facultad de Farmacia, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos; 4 Departamento de Farmacobiología, Sede Sur del Centro de Investigación y de Estudios Avanzados del IPN, México * E-mail: myrnadeciga@hotmail.com ABSTRACT Three palmitic acid derivatives were synthesized and evaluated as a potential platform for antinociceptive drug development. Female Swiss Webster mice were given N-(4-Methoxy-2-nitrophenyl)hexadecanamide (1), 2-amino-3-(palmitoylamino)benzoic acid (2) or 4- amino-3-(palmi-toylamino)benzoic acid (3) orally in doses of 10-100 mg/kg. The animals were tested for nociception using the hot plate and abdominal constriction response (writhing) tests. Compound 1 generated a dose-dependent antinociceptive effect, reflected by longer latencies (paw-lick and escape responses) and a decrease in writhing. Morphine (1.5-6 mg/kg, p.o.) and diclofenac (10-100 mg/kg, p.o.) were used as positive controls, respectively. Compounds 2 and 3 were less active in both nociceptive tests. The antinociception provoked by compound 1 was partially blocked by naloxone (1 mg/kg, i.p.) suggesting that this pharmacological effect could be due to the activation of µ-opioid receptors. N-(4-Methoxy-2-nitrophenyl)hexadecan- amide showed antinociceptive effects in both nociceptive tests suggesting the possibility that this compound may define a new type of antinociceptive. INTRODUCTION Palmitic acid (hexadecanoic acid), is one of the most common saturated fatty acids found in animals and plants. As its name indicates, it is a major component of the oil from palm trees (palm oil and palm kernel oil). Butter, cheese, milk and meat also contain this fatty acid. Palmitic acid is the first fatty acid produced during lipogenesis (fatty acid synthesis) and from which longer fatty acids can be produced. Palmitoyletanolamide (PEA) is a derivate of palmitic acid, it has been shown to inhibit peripheral inflammation [1-2] and mast-cell degranulation [3], as well as to exert neuroprotective [4-5] and antinociceptive [6] effects in rats and mice. One of its structural analogs, anandamide (arachidonoyl- ethanolamide), serves as an endogenous ligand for cannabinoid receptors. It has been demonstrated that systemic application of anandamide produces analgesia in a number of acute and inflammatory pain models [6-8]. However, systemic administration of cannabinoids have low efficacy due to their rapid degradation. In the present study, we examined the effects of three new palmitic acid derivatives (N-(4- Methoxy-2-nitrophenyl)hexa-decanamide (1), 2- amino-3-(palmitoylamino)benzoic acid (2) and 4- amino-3-(palmitoylamino)benzoic acid (3) on antin- ociceptive effect in an animal model of pain. 3 O N H NH 2 COOH O N H NH 2 COOH 2 O N H NO 2 O 1 3 O N H NH 2 COOH O N H NH 2 COOH 2 3 O N H NH 2 COOH O N H NH 2 COOH 2 O N H NO 2 O 1 O N H NO 2 O 1 Figure 1. Synthetic derivatives of palmitic acid: 1) N-(4-Methoxy-2-nitrophenyl)hexadecanamide; 2) 2-amino-3-(palmitoylamino)benzoic acid and 3) 4-amino-3-(palmitoylamino)benzoic acid. MATERIALS AND METHODS Experiments were performed on female Swiss albino mice (25-30 g) from CINVESTAV. All experiments followed the Guidelines on Ethical Standards for Investigation of Experimental Pain in Animals [9] and Mexican Official Norm for Animal Care and Handling (NOM-062-ZOO-1999). Efforts were made to minimize animal suffering and to reduce the number of animals used. Twelve hours before experiments, food was withheld, but animals had free access to drinking water. At the end of the experiment, animals were euthanized. No side effects were observed in any of the animal groups studied. Drugs: Morphine and diclofenac were purchased from Laboratories Pisa (Mexico City) and Merck (Mexico City) respectively. Naloxone was obtained from Sigma (St. Lous, MO). Compounds 1-3 were synthesized in our laboratory. Morphine, diclofenac, naloxone, and test compounds were suspended in vehicle (Tween-80, 2% in saline). Acetic acid-induced writhing : The writhing test was performed in mice as previously described [10] Compounds 1-3 (10, 30 and 100 mg/kg) were administered orally 40 min before intraperitoneal injection of 0.6% acetic acid. Control animals received vehicle or the non-steroidal anti-inflammatory drug diclofenac (10, 30 and 100 mg/kg, p.o.) as a positive control. Animals were then placed in an observation box, and the abdominal constrictions were counted cumulatively over a period of 30 min. A significant reduction in the number of abdominal constrictions was considered antinociception. 75