Biomedical subjects

Giangthy N Ton

Publications and source records attributed to Giangthy N Ton.

3 recordsLinked to original sources

PubMed2005

Methoxypoly(ethylene glycol)-conjugated carboxypeptidase A for solid tumor targeting: part I: synthesis and characterization.

In vivo efficacy of novel anticancer agents has been hindered by the inability to deliver effective concentrations of drugs to tumors. The use of macromolecules such as antibodies and polymers for enzyme delivery to tumors has revealed that catalyzing the conversion of a nontoxic prodrug into its cytotoxic form can generate an effective level of cytotoxic agents at tumor sites. This study primarily focuses on the synthesis and characterization of methoxypoly(ethylene glycol)-modified carboxypeptidase A (CPA) for solid tumor targeting. The molecular weight of CPA has been successfully altered from 35 to 40-50 kDa via attachment of a defined number of mPEG moieties. Relatively pure mPEG-CPA conjugates containing one, two, and three mPEG chains were obtained at preparative scale quantities through controlled PEGylation followed by fractionation that involved size-exclusion chromatography. An enhancement in kinetic properties including k(cat) and k(cat)/K(m) towards hippuryl-L-phenylalanine (hipp-L-phe) was observed in mPEG-CPA conjugates. An increase in the V(m) appeared to be responsible for this enhancement. The attachment of mPEG to CPA substantially improved the stability of the enzyme with respect to the specific peptidase activity toward the model substrate. This finding is particularly important in the development of a novel CPA/methotrexate-alpha-peptide system in solid tumor chemotherapy.

Animals↗

PubMed2005

Methoxypoly(ethylene glycol)-conjugated carboxypeptidase A for solid tumor targeting: part II: pharmacokinetics and biodistribution in normal and tumor-bearing rodents.

We have evaluated effects of mPEG modification on pharmacokinetic properties of carboxypeptidase A (CPA) in normal rats. Attachment of two or three mPEG chains to CPA resulted in the generation of mPEG2-CPA and mPEG3-CPA analogs with significantly enhanced plasma half-lives, especially during the distribution phase. Moreover, the assessment of real-time whole-body kinetics in CT26 tumor-bearing mice showed both mPEG2-CPA and mPEG3-CPA exhibited increased body retention at 48 h post-injection. In addition, tumor localization of mPEG3-CPA at 72 h was visualized and confirmed by fusion of the gamma-scintigraphy and microCT data sets. Results from the imaging studies support our hypothesis of a correlation between tumor uptake and enhanced circulatory half-life. Tissue distribution data indicated the combination of increased tumor extravasation and effective renal elimination observed with mPEG2-CPA at 48 h following administration led to the highest observed tumor-to-blood ratio of 4.8:1. Although the total concentration of mPEG3-CPA accumulated in tumor was higher than that of mPEG2-CPA and CPA at predetermined time intervals, a higher tumor-to-blood ratio was not obtained owing to a higher level of blood activity. Clearly, the attachment of an appropriate number of mPEG chains can facilitate tumor localization as effectively as can the use of a tumor-specific antibody.

Adenocarcinoma↗

PubMed2004

Substrate specificity for 4-thiouridine modification in Escherichia coli.

The biosynthesis of 4-thiouridine (s4U) in Escherichia coli tRNA requires the action of both the thiamin pathway enzyme ThiI and the cysteine desulfurase IscS. IscS catalyzes sulfur transfer from l-cysteine to ThiI, which utilizes Mg-ATP to activate uridine 8 in tRNA and transfers sulfur to give s4U. In this work, we show through deletion analysis of unmodified E. coli tRNA(Phe) that the minimum substrate for s4U modification is a mini-helix comprising the stacked acceptor and T stems containing an internal bulged region. The size of the bulged loop must be at least 4 nucleotides and contain the target uridine as the first nucleotide. Replacement of the T loop sequence with a tetraloop in the deletion substrate increases activity and shows that the TpsiC primary sequence is not a recognition element. An unmodified tRNA(Phe) transcript in which the 3'-terminal ACCA sequence is removed to give a blunt terminus has <0.1% activity, although the addition of a single overhanging base essentially restores activity. In addition, reducing the distance of the 3' terminus relative to U8 by as little as 1 bp severely impairs activity. By dissecting a minimal RNA substrate in the T loop region, a two-piece system consisting of a substrate RNA and a "guide" RNA is efficiently modified. Our results indicate that outside of the modified U8, there is no primary sequence requirement for substrate recognition. However, the secondary and tertiary structure restrictions appear sufficient to explain why s4U modification is limited in the cell to tRNA.

Bacterial Proteins↗