Cardiovascular Characterization of Pyrrolo[2,1-d][1,5]benzothiazepine Derivatives Binding Selectively to the Peripheral-Type Benzodiazepine Receptor (PBR):  From Dual PBR Affinity and Calcium Antagonist Activity to Novel and Selective Calcium Entry Blockers

Abstract

The synthesis and cardiovascular characterization of a series of novel pyrrolo[2,1-d][1,5]-benzothiazepine derivatives (54-68) are described. Selective peripheral-type benzodiazepine receptor (PBR) ligands, such as PK 11195 and Ro 5-4864, have recently been found to possess low but significant inhibitory activity of L-type calcium channels, and this property is implicated in the cardiovascular effects observed with these compounds. In functional studies both PK 11195 (1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxa mide) and Ro 5-4864 (4'-chlorodiazepam) did not display selectivity between cardiac and vascular tissue. Therefore, several 7-(acyloxy)-6-arylpyrrolo[2,1-d][1,5]benzothiazepines, potent and selective peripheral-type benzodiazepine receptor ligands recently developed by us (3, 7-20), were subjected to calcium channel receptor binding assay. Some of these compounds showed an unexpected potency in displacing the binding of [3H]nitrendipine from L-type calcium channels, much higher than that reported for PK 11195 and Ro 5-4864 and equal to or higher than that of reference calcium antagonists such as verapamil and (+)-cis-diltiazem. Specifically, in rat cortex homogenate, our prototypic PBR ligand 7-acetoxy-6-(p-methoxyphenyl)pyrrolo[2,1-d][1,5]benzothiazepine (3) showed an IC50 equal to 0.13 nM for inhibition of [3H]nitrendipine binding. Furthermore, in functional studies this compound displayed a clear-cut selectivity for cardiac over vascular tissue. Comparison of calcium antagonist activity on guinea pig aorta strips with the negative inotropic activity, determined by using isolated guinea pig left atria, revealed that 3 displayed higher selectivity than the reference (+)-cis-diltiazem. Thus, the pyrrolobenzothiazepine 3 might represent a new tool for characterizing the relationship between the PBR and cardiac function. Furthermore, we have also investigated the structural dependence of binding to PBR and L-type calcium channels, and this study allowed us to identify a new class of potent calcium channel blockers selective for cardiac over vascular tissue, with no affinity for PBR. A number of structure-activity relationship trends have been identified, and a possible explanation is advanced in order to account for the observed differences in selectivity. Three structural features, namely, (i) the saturation of the C(6)-C(7) double bond, with a consequent higher molecular flexibility, (ii) the presence of a substituent in the benzofused ring, and (iii) a basic side chain at C-10 of the pyrrolobenzothiazepine ring system, were found to be responsible for potent L-type calcium channel antagonism and clear-cut selectivity for cardiac over vascular tissue. Among the synthesized compounds the pyrrolobenzothiazepine 62 was found to be the most promising selective calcium channel blocker. Additionally, the molecular structure determination of the key intermediate 48 by X-ray diffraction, molecular modeling, and NMR analysis is reported.

BubR1 Is Required for a Sustained Mitotic Spindle Checkpoint Arrest in Human Cancer Cells Treated with Tubulin-Targeting Pyrrolo-1,5-Benzoxazepines

“…PBOX-6 (7-[(dimethylcarbamoyl) oxy]-6-(2-naphthyl)pyrrolo-[2,1-d][1,5]benzoxazepine) and PBOX-15 (4-acetoxy-5-(-1-(naphthyl)naphtho [2,3-b]pyrrolo [2,1-d] [1,4]oxazepine) were synthesized as described previously (Campiani et al, 1996) and dissolved in ethanol. Structures of the PBOX compounds were described by Mulligan et al (2006).…”

Structural Requirements to Obtain Highly Potent and Selective 18 kDa Translocator Protein (TSPO) Ligands

“…In a separate study, the authors described the synthesis and cardiovascular characterization of a number of previously developed [70,71] or newly synthesized pyrrolo[2,1-d] [1,5]-benzothiazepine derivatives [97]. Indeed, selective TSPO ligands, such as Ro 5-4864 (2) and PK 11195 (3), have been found to possess low but significant inhibitory activity of L-type calcium channels, producing beneficial cardiovascular effects.…”

“…Subsequently, to evaluate the consistency of the SAR trends, the basic structure of compounds (95)(96)(97) was subjected to structural modifications in three of the four postulated pharmacophoric groups ( 1, LA, FRA), to give compounds 98-102, Table 9 [111]. Although a general decrease in TSPO affinity was observed with respect to the parent compounds, almost all the new derivatives showed TSPO affinities in the nanomolar range, with a modulation suitable for a SAR analysis, that find correlations with previous reports, Table 9.…”

Involvement of endothelium-dependent and -independent mechanisms in midazolam-induced vasodilation

“…To explain these results, it has been suggested that Ro5-4864 also possesses a low affinity for the voltage-operated calcium channels and that at micromolar concentrations, it can probably compete for the same BDZ-binding site on voltage-operated calcium channels. 32,33 These observations suggest that at high concentrations, midazolam possesses the calcium-channel antagonist property of acting directly on voltage-operated calcium channels. 12,17,[34][35][36][37] The calcium-antagonist property of midazolam supports the endothelium-independent mechanism of midazolam-induced vasodilation that we observed in this study.…”