Helimeric porphyrinoids: Stereostructure and chiral resolution of meso -tetraarylmorpholinochlorins

The synthesis and chiral resolution of free-base and Ni(II) complexes of a number of derivatives of meso-tetraphenylmorpholinochlorins, with and without direct β-carbon-to-o-phenyl linkages to the flanking phenyl groups, is described. The morpholinochlorins, a class of stable chlorin analogues, were synthesized in two to three steps from meso-tetraphenylporphyrin. The conformations and the relative stereostructures of a variety of free-base and Ni(II) complexes of these morpholinochlorins were elucidated by X-ray diffractometry. Steric and stereoelectronic arguments explain the relative stereoarray of the morpholino-substituents, which differ in the free-base and Ni(II) complexes, and in the monoalkoxy, β-carbon-to-o-phenyl linked morpholinochlorins, and the dialkoxy derivatives. The Ni(II) complexes were all found to be severely ruffled whereas the free-base chromophores are more planar. As a result of the helimeric distortion of their porphyrinoid chromophores, the ruffled macrocycles possess a stable inherent element of chirality. Most significantly, resolution of the racemic mixtures was achieved, both by classical methods via diastereomers and by HPLC on a chiral phase. Full CD spectra were recorded and modeled using quantum-chemical computational methods, permitting, for the first time, an assignment of the absolute configurations of the chromophores. The report expands the range of known pyrrole-modified porphyrins. Beyond this, it introduces large chiral porphyrinoid π-systems that exist in the form of two enantiomeric, stereochemically stable helimers that can be resolved. This forms the basis for possible future applications, for example, in molecular-recognition systems or in materials with chiroptic properties. © 2011 American Chemical Society

In vitro and in vivo evaluation of doxorubicin conjugates with the divalent peptide E-[c(RGDfK)2] that targets integrin alphavbeta3

Integrins, especially integrin alpha vbeta 3, are attractive receptors for vascular targeting strategies. Recently, a divalent RGD peptidomimetic, E-[c(RGDfK) 2], has been described that demonstrates increased uptake in human ovarian carcinoma OVCAR-3 xenograft tumors. Inspired by these results, we set out to develop doxorubicin conjugates with E-[c(RGDfK) 2] by binding two different maleimide derivatives of doxorubicin to E-[c(RGDfK) 2] that was thiolated with iminothiolane. In this way, two water-soluble derivatives were obtained, E-[c(RGDfK) 2]-DOXO-1 and E-[c(RGDfK) 2]-DOXO-2. In E-[c(RGDfK) 2]-DOXO-1, doxorubicin was bound to the peptide through a stable amide bond, and in E-[c(RGDfK) 2]-DOXO-2, a MMP-2/MMP-9 cleavable octapeptide was introduced between doxorubicin and the peptide. The rationale for a MMP-2/MMP-9-cleavable linker was that MMP-2 and MMP-9 bind to integrin alpha vbeta 3 and both are overexpressed in tumor vasculature. In addition, analogous control doxorubicin-containing peptides bearing c(RADfK) that does not bind to integrin alpha vbeta 3 were synthesized, i.e., c(RADfK)-DOXO-1 and c(RADfK)-DOXO-2. Whereas E-[c(RGDfK) 2]-DOXO-2 was cleaved effectively by MMP-2 and in OVCAR-3 tumor homogenates releasing a doxorubicin-tetrapeptide or doxorubicin as the final cleavage product, no release of doxorubicin was observed for E-[c(RGDfK) 2]-DOXO-1. Proliferation of HUVEC in the presence of MMP-2-cleavable doxorubicin-containing peptides exhibited 6- to 10-fold increased inhibition compared to the amide-linked doxorubicin-containing peptides. In addition, inhibition of HUVEC sprouting during a 24 h exposure was approximately 3-fold stronger for E-[c(RGDfK) 2]-DOXO-2 and 20-fold stronger for the reference peptide conjugate c(RADfK)-DOXO-2 than for doxorubicin alone. In vivo studies in an OVCAR-3 xenograft model demonstrated no or only moderate antitumor efficacy for either E-[c(RGDfK) 2], E-[c(RGDfK) 2]-DOXO-1, E-[c(RGDfK) 2]-DOXO-2, or c(RADfK)-DOXO-2, even at doses of 3 x 24 mg/kg doxorubicin equivalents, compared to an improved antitumor effect for doxorubicin at 2 x 8 mg/kg