Findings questioning the involvement of Sigma-1 receptor in the uptake of anisamide-decorated particles.

Anisamide is a small benzamide previously suggested as a tumor-targeting ligand for nanocarriers and it has been shown to enhance tumor uptake in vitro as well as in vivo when grafted on the nanoparticle surface. Anisamide has been hypothesized to interact with the Sigma-1 receptor, based on the binding of larger benzamides, which contain anisamide in their structure, to this receptor. However, the interaction between anisamide and Sigma-1 receptor has never been thoroughly studied. We developed fluorescent PEGylated particles decorated with anisamide, which were preferentially taken up in vitro by melanoma cells compared to macrophages. The anisamide-decorated particles were used to study their interaction with the Sigma-1 receptor. The absence of competition of Sigma-1 receptor ligands for the particle uptake was a first indication that the receptor might not be involved in the uptake process. In addition, the extent of particle uptake did not correlate with the levels of cellular expression of Sigma-1 receptor in the cell models tested. Immunostaining of the receptor on melanoma cells revealed intracellular localization, indirectly excluding the possibility of anisamide binding to the receptor when grafted on the particles. All these data question the previously suggested Sigma-1 receptor-mediated uptake of the anisamide-decorated particles, a finding which may have an impact on the use of anisamide as a targeting ligand.

Increases in rat striatal extracellular dopamine and vacuous chewing produced by two sigma receptor ligands.

The present studies were carried out to analyze the neurochemical and behavioral effects of peripheral sigma ligand administration in the rat. Based upon previous studies which showed an increase in turning behavior following unilateral intranigral administration of sigma ligands, we determined the effects of two sigma ligands, 1,3-di-o-tolylguanidine (DTG) and (+)-pentazocine, on extracellular dopamine levels in the rat striatum. Dopamine levels were monitored via microdialysis in awake freely moving animals following i.p. injection of the ligands. Both DTG (1 and 3 mg/kg) and (+)-pentazocine (10 mg/kg) produced a significant (30-50%) increase in extracellular dopamine. Given the relatively high concentration of sigma receptors in brain nuclei involved in facial and mouth movements, we have also determined the effects of the two sigma ligands on facial movements. Both ligands produced a significant increase in vacuous chewing movements, suggesting that studies on the consequences of sigma receptor activation may have relevance to animal models of human dystonia and/or dyskinesia.

[Mechanisms of action of antidepressants: new data from Escitalopram]. / De nouvelles données pour appréhender les mécanismes d'action des antidépresseurs: l'apport du Escitalopram.

A first improvement in the treatment of depression was achieved in 1970-80 with the development of selective serotonin reuptake inhibitors (SSRI) because these drugs, which are as potent antidepressants as the tricyclics, are devoid of most of the secondary effects of the latter drugs (orthostatic hypotension, weight gain, dry mouth, etc, mainly caused by their capacity to block alpha1-adrenergic, H1 histaminergic and muscarinic receptors). However, SSRI did not solve all the problems inherent to the treatment of depression because (i) approximately 30% of depressed patients do not respond to these drugs, and (ii) their antidepressant effect becomes really significant only after 3-4 weeks of treatment, like that observed with tricyclics. A further improvement in the development of antidepressant drugs has recently been made with the synthesis of the S enantiomer of citalopram, called Escitalopram. Indeed, this active enantiomer is the most selective among all SSRI available to date, including citalopram. In addition, the potency of Escitalopram to inhibit serotonin reuptake (K(i)=2,1 nM) and to induce antidepressant-like effects in relevant animal paradigms (forced swimming test; chronic mild stress; stress-induced ultrasonic vocalization) is markedly increased as compared with citalopram and other SSRI. In particular, in the forced swimming test, which is especially relevant for assessing the potential antidepressant properties of drugs, Escitalopram was shown to be at least 15 fold more potent than any other SSRI to delay helplessness-induced immobility of rats. Even more interestingly, under chronic treatment conditions, Escitalopram was found to be significantly more rapid than any other antidepressant (tricyclics such as imipramine, SSRI such as fluoxetine) to restore sucrose intake in rats subjected to chronic mild stress, suggesting a reduced delay in its antidepressant action. This was indeed fully confirmed in humans as only 1-2 weeks of treatment with Escitalopram was enough to significantly reduce MADRS score in depressed subjects, compared to 3-4 weeks with any other antidepressant drug. These unique properties led to further investigations of the pharmacological profile of Escitalopram. It thus appeared that, at equipotent doses, the S enantiomer was significantly more efficient than citalopram (racemate) to increase the extracellular levels of serotonin within the frontal cortex of freely moving rats bearing a locally implanted microdialysis probe. Further experiments showed that R-citalopram counteracted the capacity of Escitalopram to enhance extracellular 5-HT levels, thereby explaining why the racemate had only a limited action in this regard. In addition, behavioural studies (stress-induced ultrasonic vocalization test) also showed that R-citalopram exerts effects opposite to those (antidepressant--and anxiolytic--like effects) of Escitalopram. The reason for these differences between the two enantiomers might concern the secondary molecular targets at which citalopram acts, but with affinities at least two orders of magnitude less than for the serotonin transporter. Indeed, R-citalopram has a 7-10-fold higher affinity for H1 histaminergic (K(i)=180 nM) and alpha1-adrenergic (K(i)=560 nM) receptors than Escitalopram (respective K(is) > or = 2 000 nM), and this difference might contribute not only to the better selectivity of the latter enantiomer for its therapeutically relevant target (i.e. the serotonin transporter) but also to its improved capacity to enhance central 5-HT neurotransmission. On the other hand, the global affinity of Escitalopram (K(i)=200-430 nM) for both subtypes of sigma receptors (sigma1 and sigma2) is higher than that of R-citalopram (and of the racemate citalopram; K(i)=200-1 500 nM), and this might also strengthen the antidepressant and anxiolytic effects of the S enantiomer because behavioural studies showed that selective sigma1 and sigma2 agonists are endowed with both antidepressant--and anxiolytic-like properties in relevant animal models. However, to date, the exact nature (agonist or antagonist) of the action of Escitalopram at sigma receptors is not known yet, and this question has to be addressed in future investigations. Altogether, these data open novel perspectives for both a better treatment of depressive disorders and a better knowledge of the neurobiological mechanisms underlying antidepressant therapy, and, possibly, depression itself.

Anisamide-targeted stealth liposomes: a potent carrier for targeting doxorubicin to human prostate cancer cells.

Certain human malignancies including prostate cancer overexpress sigma receptor, a membrane bound protein that binds haloperidol and various other neuroleptics with high affinity. An anisamide derivatized ligand possesses high affinity for sigma receptors and we hypothesized that its incorporation into the liposomes encapsulating doxorubicin (DOX) can specifically target and deliver the drug to prostate cancer cells that overexpress sigma receptors. A polyethylene glycol phospholipid was derivatized with an anisamide ligand, which was then incorporated into the DOX-loaded liposome. The resulting anisamide-conjugated liposomal DOX showed significantly higher toxicity to DU-145 cells than non-targeted liposomal DOX, the IC50 being 1.8 microM and 14 microM respectively. The cytotoxicity of the targeted liposomal DOX, however, was significantly blocked by haloperidol, suggesting that the enhanced cytotoxicity was specifically mediated by the sigma receptors. Fluorescence imaging studies after intravenous (i.v.) administration showed that incorporation of anisamide into liposomes significantly improved their accumulation into the tumor. A weekly injection of the targeted liposomal DOX for 4 weeks at a dose of 7.5 mg/kg led to a significant growth inhibition of established DU-145 tumor in nude mice with minimal toxicity. Free DOX was effective, but associated with significant toxicities. The present study is the first to demonstrate the use of small molecular weight ligand for mediating efficient targeting of liposomal drugs to sigma receptor expressing prostate cancer cells both in vitro and in vivo.