Thrombospondin-1 mimetic peptide inhibitors of angiogenesis and tumor growth: design, synthesis, and optimization of pharmacokinetics and biological activities.

The heptapeptide 1, NAc-Gly-Val-DIle-Thr-Arg-Ile-ArgNHEt, a structurally modified fragment derived from the second type-1 repeat of thrombospondin-1 (TSP-1), is known to possess antiangiogenic activity. However, therapeutic utility could not be demonstrated because this peptide has a very short half-life in rodents. To optimize the PD/PK profile of 1, we initiated a systematic SAR study. The initial structural modifications were performed at positions 5 and 7 of peptide 1 and at the N- and C-termini. Out of several hundred peptides synthesized, the nonapeptide 5 (ABT-526) emerged as a promising lead. ABT-526 inhibited VEGF-induced HMVEC cell migration and tube formation in the nanomolar range and increased apoptosis of HUAEC cells. ABT-526 showed acceptable PK in rodents, dog, and monkey. ABT-526, when incorporated in an angiogenic pellet implanted in the rat cornea at 10 microM, reduced neovascularization by 92%. Substitution of DalloIle in place of DIle in ABT-526 provided nonapeptide 6 (ABT-510), which was 30-fold less active than ABT-526 in the EC migration but 20-fold more active in the tube formation assay. In comparison to ABT-526, ABT-510 has increased water solubility and slower clearance in dog and monkey. Radiolabeled ABT-510 demonstrated saturable binding to HMVEC cells at 0.02-20 nM concentrations and was displaceable by TSP-1. ABT-510 and ABT-526 were shown to significantly increase apoptosis of HUAEC cells. ABT-510 was effective in blocking neovascularization in the mouse Matrigel plug model and inhibited tumor growth in the mouse Lewis lung carcinoma model. Previous studies had shown that ABT-510 was effective in inhibiting the outgrowth of murine melanoma metastases in syngeneic mice and in blocking the growth of human bladder carcinoma implanted in nude mice. It had been also shown that ABT-510 could regress tumor lesions in pet dogs or cause unexpected stabilization of the disease in advanced canine cancer. ABT-526 and ABT-510 are the first compounds in the class of potent inhibitors of angiogenesis that mimic the antiangiogenic function of TSP-1. ABT-510 is currently in phase II clinical studies.

Implication of P450-metabolite complex formation in the nonlinear pharmacokinetics and metabolic fate of (+/-)-(1'R*,3R*)-3-phenyl-1-[(1',2',3',4'-tetrahydro-5',6'-methylene-dioxy-1'-naphthalenyl) methyl] pyrrolidine methanesulfonate (ABT-200) in dogs.

Following a single oral or intravenous administration of the R,R(+) and S,S(-) (14)C-pseudoracemate of (+/-)-(1'R*,3R*)-3-phenyl-1-[(1',2',3',4'-tetrahydro-5',6'-methylene-dioxy-1'-naphthalenyl) methyl] pyrrolidine methanesulfonate (ABT-200/I) to dogs, a total of six (R,R[+]) and eight (S,S[-]) metabolites were identified by high-pressure liquid chromatography/mass spectral techniques. Greater than 99% of the dose was eliminated as metabolites indicating that the clearance of I was predominantly metabolic. The catechol was the major excreted metabolite (fecal), whereas the urine and bile predominantly contained metabolites resulting from secondary biotransformation of the catechol via O-methylation, glucuronidation, and sulfation. After a single 12 mg/kg oral dose of racemic I to dogs, the mean area under the plasma curve (AUC(0-24h)) averaged 4.55 micro g. h/ml, with an apparent plasma clearance value of 2.70 l/h. kg. After 14 daily doses, the apparent plasma clearance was 3.5-fold lower (0.78 l/h. kg) and the AUC(0-24h) about 4-fold higher (18.58 micro g. h/ml). Isolation of liver microsomes from these animals indicated that a cytochrome p450 (p450)-metabolite complex (MI complex) was formed in the liver after both single and multiple dosing. The mean concentration of the MI complex 24 h after a single dose averaged 31 pmol/mg of microsomal protein, whereas the amount in the animals given multiple doses of I averaged 163 pmol/mg. There was a positive correlation (R(2) = 0.993) between the plasma AUC for I and the amount of the MI complex found in the liver of each animal. Formation of the MI complex was demonstrated in vitro in control dog liver microsomes, was NADPH-dependent, and was dissociated from p450 with 2-methylbenzimidazole. In vitro preincubation studies indicated that I was a mechanism-based inhibitor and that formation of the complex inhibited catechol formation. These results demonstrate that the liver p450s that metabolize I form an inhibitory MI complex after in vivo administration in dogs. Formation of the complex increases during multiple dosing and inhibits the enzymes from further metabolism of I resulting in nonlinear pharmacokinetics.

[3H]A-317491, a novel high-affinity non-nucleotide antagonist that specifically labels human P2X2/3 and P2X3 receptors.

A-317491 is a potent and selective antagonist of P2X3 and P2X(2/3) receptors. In the present studies, the ability of [3H]A-317491 to label recombinant human P2X(2/3) and P2X(3) receptors was characterized. Using membranes prepared from 1321N1 cells expressing P2X(2/3) receptors, [3H]A-317491 specifically labeled high-affinity (Kd = 0.9 nM) recognition sites. High-affinity [3H]A-317491 binding was not detected in membrane preparations from native 1321N1 cells or cells expressing homomeric P2X1, P2X2, or P2X3 receptors. Specific [3H]A-317491 P2X3 receptors could only be reliably detected following treatment of intact P2X3 receptor-expressing cells with apyrase (1 U/ml) both before and during membrane preparation. Under these conditions, [3H]A-317491 also labeled high-affinity (Kd = 9 nM) binding sites. Lower affinity binding components (Kd values of 87-790 nM) were detected in both assays using higher ligand concentrations that likely represent nonfunctional recognition sites. [3H]A-317491 binding to both P2X(2/3) and P2X3 receptors was reversible, and ligand kinetic studies provided similar estimates of the high-affinity binding constants. Potent P2X3 receptor agonists 2-methylthio-ATP, 2,3-O-(4-benzoylbenzoyl)-ATP, and alpha,beta-methylene adenosine triphosphate also potently inhibited specific [3H]A-317491 binding to both P2X(2/3) and P2X3 receptors. The pharmacological profile for P2X receptor antagonists to inhibit [3H]A-317491 binding to P2X(2/3) and P2X3 receptors was highly correlated (r = 0.98, P < 0.05), and a similar rank order of potency was observed for blockade of P2X(2/3) receptor-mediated calcium influx. These data demonstrate that [3H]A-317491 is the first useful radioligand for the specific labeling of P2X3-containing channels.