Many new chemotherapeutic agents are under preclinical investigation and, despite efforts to more selectively target cancer cells, limitations such as toxicity and inherent resistance are often encountered. Therefore, alternative strategies are needed to treat cancer and overcome such limitations. We describe novel cyclohexylpiperazine derivatives, designed as mixed affinity ligands for sigma (s) receptors and human D8– D7 sterol isomerase (HSI) ligands, which also exhibit P-glycoprotein (P-gp) inhibitory activity, with the aim of exploiting the antiproliferative effects mediated by s and HSI sites while overcoming P-gp-mediated resistance. All of the compounds displayed high affinities for s receptors and HSI sites, P-gp inhibitory activity, and s2 receptor agonist antiproliferative activity. Antiproliferative activity was also tested in PC-3 cells to establish s1 and HSI contribution. Compound cis-11, which displayed the best antiproliferative and P-gp inhibitory activities, was coadministered with 0.1 mm doxorubicin in MDCK-MDR1 cells. Compound cis-11 caused 70% and 90% cell death when coadministered at 30 mm and 50 mm, respectively. When administered alone, cis-11 resulted in 50% cell death, demonstrating its single agent antitumor properties in a tumor cell line overexpressing P-gp.

σ(2) Receptor research is receiving increasing interest with regard to the potential of σ(2) proteins as targets for tumor therapy and diagnosis. Nevertheless, knowledge about the σ(2) receptor is far from conclusive. The paucity and modest affinity of known σ(2) antagonists represent one of the limitations to σ(2) receptor research. Previous studies of the high-affinity σ(2) agonist 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)-n-propyl]piperazine 4 (PB28) suggested that a decrease in lipophilicity might lead to σ(2) ligands devoid of antiproliferative activity (potential σ(2) antagonists). With the aim of producing σ(2) receptor antagonists, we replaced the tetralin nucleus of compound 4 with a 2-aminopyridine moiety. A series of compounds with high affinity for both σ subtypes and with no antiproliferative activity in various cells (mouse HT-22, human SK-N-SH, MCF-7wt, and MCF-7σ(1) ) were obtained. The effect on Ca(2+) mobilization was investigated for high-affinity compounds 18 and 4, which showed opposite effects. All of the data support the new 2-aminopyridines as high-affinity σ ligands with σ(2) antagonist and σ(1) agonist activity, and, despite the lack of significant σ(2) versus σ(1) selectivity, these novel compounds may be better tools for σ receptor research than the known low-affinity σ(2) antagonists.

1-Cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine 1 (PB28) and 2- Methoxy-5-methyl-N-[4-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)butyl]benzamide 2 (RHM-1) represent leads for tumor diagnosis, given their high affinity at sigma2 receptors. With the purpose of obtaining good candidates for s2 PET tracers development, hybrid structures between 1 and 2 were designed. Excellent sigma1/sigma2 selectivities were reached when 6,7-dimethoxytetrahydroisoquinolinewas linked to an omethoxy substituted arylamide (11a, 12a, 15a), and for these benzamides an intramolecular H-bond in the active conformation at the s sites, was hypothesized. However these excellent s2 ligands were accompanied by interaction with P-gp, which may limit their use as sigma2 receptor PET agents when tumors overexpress P-gp. Compound 15a whose P-gp interaction was just moderate represents an interesting tool for the development of sigma2 PET tracers useful in tumors overexpressing P-gp.

A series of polymethyl-substituted piperidines linked to either a 6-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl or a 6-methoxynaphthalen-1-yl moiety was generated with the aim of verifying a previously generated hypothesis: tetralin and naphthalene nuclei confer opposite activity at the 1 receptor. Compounds 6, 9 and 10 displayed appreciable affinity at both  subtypes, but none of the novel compounds displayed significant antiproliferative activity in MCF7wt and MCF71 cell lines. The effect on bradikynin-triggered Ca2+ mobilization was studied as a methodology to suggest sigma receptors mediated activity.

Despite the promising potentials of σ2 receptors in cancer therapy and diagnosis, there are still ambiguities related to the nature and physiological role of the σ2 protein. With the aim of providing potent and reliable tools to be used in σ2 receptor research, we developed a novel series of fluorescent σ2 ligands on the basis of our previous work, where high-affinity σ2 ligand 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)-n-propyl]piperazine (1, PB28) was used as the pharmacophore. Compared to the previous compounds, these novel ligands displayed improved fluorescence and σ2 binding properties, were σ2-specifically taken up by breast tumor cells, and were successfully employed in confocal microscopy. Compound 14, which was the best compromise between pharmacological and fluorescent properties, was successfully employed in flow cytometry, demonstrating its potential to be used as a tool in nonradioactive binding assays for studying the affinity of putative σ2 receptor ligands.

To combat the emergence of drug-resistance in tumors novel strategies are urgently needed. With this in mind we designed a novel class of thiosemicarbazones able to target simultaneously 2 receptors and P-glycoprotein efflux pump while chelating metals such as Iron. The combined effect of these targets would lead to the activation of multiple pathways to which resistant tumors are sensitive. Indeed, most of the novel thiosemicarbazones displayed antiproliferative activity in both parent (MCF7 breast adenocarcinoma and A549 lung carcinoma) and corresponding doxorubicine-resistant cells (MCF7dx and A549dx). A few compounds emerged for their potent antiproliferative activity or for their more potent effect in doxorubicine-resistant cells than in the parent ones, while other compounds emerged for their remarkable P-gp modulatory activity. These results pave the way for further studies on these targets in the oncology field, while the availability of promising molecules for resistant tumors treatment that warrant deeper investigations was increased.

Starting from two carbocyclic analogs, a series of 3,3-dimethylpiperidine derivatives was prepared and tested in radioligand binding assays at r1 and r2 receptors, and at D8–D7 sterol isomerase (SI) site. The novel compounds mostly bear heterocyclic rings or bicyclic nucleus of differing lipophilicities. Compounds 18a and 19a,b demonstrated the highest r1 affinity (Ki = 0.14–0.38 nM) with a good selectivity versus r2 binding. Among them, 18a had the lowest ClogD value (3.01) and only 19b was selective versus SI too. Generally, it was observed that more planar and hydrophilic heteronuclei conferred a decrease in affinity for both r receptor subtypes.