Toshiro Yano

Determinants for the drug release from T-0128, camptothecin analogue-carboxymethyl dextran conjugate

To improve pharmacological profiles of camptothecins (CPTs), a new macromolecular prodrug, denoted T-0128, was synthesized. This prodrug comprises a novel CPT analog (T-2513: 7-ethyl-10-aminopropyloxy-CPT) bound to carboxymethyl (CM) dextran through a Gly-Gly-Gly linker, with a molecular weight of 130 kDa. The present study was designed to elucidate the mechanisms that promote the release of linked T-2513. First, we compared the abilities of a rat liver homogenate, a cocktail of its lysosomal enzymes, and different types of pure enzymes, to liberate T-2513 from the conjugate. The releasing rate in the homogenate was very slow, but was accelerated with the lysosomes. Lysosomal cysteine proteinases, such as cathepsin B, were responsible, coupled with the results of in vitro and in vivo inhibition studies using proteinase inhibitors. The pH optimum for the cathepsin B-mediated drug release was approximately 4. This corresponds to the pH in lysosomes, suggesting lysosomotropic release. Second, to assess the effect of the length and composition of the peptidyl linker, we synthesized the conjugates with a different linker and compared the drug-releasing rates. We found that the insertion of Phe into Gly-Gly-Gly allowed various kinds of enzymes to produce a rapid cleavage, and the Gly-chain lengthening enhanced the lysosome-mediated drug release. The released T-2513 levels in the liver and tumor of the tumor-bearing rats dosed with each conjugate increased with the length of Gly linker, suggesting a good in vitro to in vivo relationship. Comparative efficacy studies of the conjugates with a different linker demonstrated that T-0128 showed the maximum efficacy against MX-1 human mammary xenograft tumors. Thus the Gly-Gly-Gly linker exploits lysosomal cathepsin B to liberate T-2513 slowly and steadily, resulting in improved therapeutic efficacy.

Structure-activity relationships of carboxymethylpullulan-peptide-doxorubicin conjugates : Systematic modification of peptide spacers

A series of carboxymethylpullulan (CMPul)-doxorubicin (DXR) conjugates bound by peptide spacers of different compositions and lengths were prepared and evaluated for their in vivo antitumor effects. Systematic study of the peptide spacers indicated that CMPul-DXR conjugates bound via appropriate dipeptide spacers were more potent than DXR.

MEN4901/T-0128, a New Camptothecin Derivative–Carboxymethyldextran Conjugate, Has Potent Antitumor Activities in a Panel of Human Tumor Xenografts in Nude Mice

Purpose: The purpose of the present study was to evaluate the antitumor activity and pharmacokinetic profile of MEN4901/T-0128 in nude mice bearing human tumor xenografts in comparison with irinotecan (CPT-11) and T-2513. Experimental Design: We have determined the antitumor activity of MEN4901/T-0128, CPT-11, and T-2513 in BALB/cA Jcl nude mice bearing human gastric (H-81), colon (H-110), lung (Mqnu-1, H-74), esophageal (H-204), liver (H-181), and pancreatic (H-48) cancer lines, which had been serially transplanted s.c. and maintained in nude mice, and characterized the pharmacokinetic profile of MEN4901/T-0128 in nude mice bearing human gastric carcinoma St-4. Results: MEN4901/T-0128 administered i.v. showed a marked antitumor activity in each of these tumor models, producing tumor shrinkage in the models of H-204 and H-181 carcinomas at its maximum tolerated dose of 80 mg/kg (expressed as T-2513) weekly for 4 weeks (q7d × 4) and tumor-shrinking or marked growth-inhibitory effects in the models of H-81, H-110, Mqnu-1, H-74, and H-48 carcinomas at 1/3 of its maximum tolerated dose (q7d × 4). Pharmacokinetic analysis showed that MEN4901/T-0128 had an extended plasma half-life with sustained tumor levels of T-2513, which may explain the superior activity of MEN4901/T-0128 in vivo. Conclusions: Because the efficacies of some drugs in this human cancer-nude mouse panel correlated well with their clinical outcomes in patients with the same type of cancers, the findings provide direct support that MEN4901/T-0128 is more efficacious than CPT-11 and is an excellent candidate for clinical trials for the treatment of solid tumors.

Synthesis and distribution study in tumor bearing-rats of oxidized carboxymethyl-D-manno-D-glucan-DXR conjugates.

Oxidized carboxymethyl-D-manno-D-glucan (MG-CM-AD)-DXR conjugates were prepared via Schiffs base formation and compared tissue distribution of the conjugates with that of DXR. The water-soluble conjugates, which had DXR contents of 11∼13% (w/w) and degrees of substitution(DS)of CM groups of 0.5∼1.0, released DXR with an increase in the DS values in 50 mM phosphate buffer(pH 7.4)at 37°C. The conjugates showed higher plasma and tumor concentration but lower heart concentration than DXR at 24 hr after the intravenous injection into Walker 256 bearingrat. The area under the concentration-time curve from 0 to 48 hr of MG-CM [0.5]-AD-DXR(DS of CM groups : 0.5) was 2 times higher on tumor, but 2 times lower on heart than that of DXR. These findings suggest that MG-CM [0.5]-AD-DXR should exhibit a higher antitumor effect and less toxicity than DXR.