New macrocyclic analogs of the natural histone deacetylase inhibitor FK228; design, synthesis and preliminary biological evaluation.

Among the natural histone deacetylase inhibitors (HDACi), the bicyclic depsipeptide macrolactone FK228 stands out for its unique chemical structure and mechanism of action. In order to expand the chemical diversity, exploiting the FK228 peculiar structure, we have synthesized a collection of 24 simplified novel analogs. A first series consists of bicyclic macrolactones, where the carboxy terminus of the natural compound was substituted by peptidomimetic aminomethylphenylacetic acid derivatives. These analogs, 7a-i, showed submicromolar cytotoxic activity, even though very low inhibitory activity against HDAC enzymes, suggesting that most probably they behave with a mechanism different from the natural compound. One of the most active members in the group, 7g, was evaluated in vivo and exhibited significant antitumor activity. This evidence supports that the activity is unrelated to HDAC inhibition and these compounds represent a novel series of promising active agents. Another analog series consists of monocyclic macrolactones, 9a-c and 10a-d which lack the disulfide bridge and bear the protected sulfur on the linear external chain; they showed similar cytotoxic activities compared to the natural compound, but proved to be very sensitive to the nature of the sulfur protection. In fact, when the sulfur was protected by an 1-octanoyl residue, like in 9b, the product displayed a one digit nanomolar activity. The results provide evidence that our approach may be followed to develop novel series of FK228 analogs.

Conjugates of a novel 7-substituted camptothecin with RGD-peptides as α(v)ß3 integrin ligands: An approach to tumor-targeted therapy.

Eight conjugates of a novel camptothecin derivative (Namitecan, NMT) with RGD peptides have been synthesized and biologically evaluated. This study focused on factors that optimize the drug linkage to the transport vector. The different linkages investigated consist of heterofunctional glycol fragments and a lysosomally cleavable peptide. The linkage length and conformation were systematically modified with the purpose to understand their effect on receptor affinity, systemic stability, cytotoxicity, and solubility of the corresponding conjugates. Among the new conjugates prepared, C6 and C7 showed high receptor affinity and tumor cell adhesion, acceptable stability in murine blood, and high cytotoxic activity (IC50 = 8 nM). The rationale, synthetic strategy, and preliminary biological results will be presented.

Novel tumor-targeted RGD peptide-camptothecin conjugates: synthesis and biological evaluation.

Five RGD peptide-camptothecin (CPT) conjugates were designed and synthesized with the purpose to improve the therapeutic index of this antitumoral drug family. New RGD cyclopeptides were selected on the basis of their high affinity to alpha(v) integrin receptors overexpressed by tumor cells and their metabolic stability. The conjugates can be divided in two groups: in the first the peptide was attached to the drug through an amide bond, in the second through a hydrazone bond. The main difference between the two spacers lies in their acid stability. Affinity to the receptors was maintained for all conjugates and their internalization into tumor cells was demonstrated. The first group conjugates showed lower in vitro and in vivo activity than the parent drug, probably due to the excessive stability of the amide bond, even inside the tumor cells. Conversely, the hydrazone conjugates exhibited in vitro tumor cell inhibition similar to the parent drug, indicating high conversion in the culture medium and/or inside the cells, but their poor solubility hampered in vivo experiments. On the basis of these results, information was acquired for additional development of derivatives with different linkers and better solubility for in vivo evaluation.

Incorporation of the unusual C(alpha)-fluoroalkylamino acids into cyclopeptides: synthesis of arginine-glycine-aspartate (RGD) analogues and study of their conformational and biological behavior.

A series of six arginine-glycine-aspartate (RGD) cyclopeptide analogues containing a C(alpha)-di- or trifluoromethylamino acid (alpha-Dfm or alpha-TfmAaa) at different positions of the ring were synthesized. All peptides were obtained in two diastereomeric forms, which were separated by HPLC. In vitro biological tests of the new cyclopeptides P were carried out in comparison with their corresponding cyclopeptides R lacking the alpha-fluoromethyl group. Five out of the six compounds P-I (containing (S)-alpha-Tfm-Aaa) showed activities in the nanomolar range, while the P-II compounds (containing (R)-alpha-Tfm-Aaa) were much less active or totally inactive. Only cyclo[RGDf-(S)-alphaTfmV] (P1-I) was found to be significantly more active than its model compound cyclo(RGDfV) (R1). The three-dimensional structure in water and DMSO was determined by NMR techniques and molecular dynamics (MD) calculations, but it was not possible to highlight significant differences in the backbone conformation of the peptides examined. Significant interproton distances, derived from nuclear Overhauser effect (NOE) experiments, were used to determine the absolute configuration of the side chains.