Synthesis and application of two reagents for the introduction of sulfhydryl groups into proteins

One-Pot Synthesis of Chicken-Feather-Keratin-Based Prodrug Nanoparticles with High Drug Content for Tumor Intracellular DOX Delivery

pH/reduction dual-triggered chicken-feather-keratin-based prodrug nanoparticles (C-PK/- SS-Hy-D NPs) were designed via a facile one-pot oxidation coupling reaction between the thiol-functional acid-labile prodrug M-Hy-D and the PEGylated keratin (PK) graft copolymer, for tumor intracellular doxorubicin (DOX) delivery. Due to the encapsulation of the pH and the reduction of the dual-responsive small prodrug D-Hy- SS-Hy-D, a high drug content of 45.8% was obtained in the proposed prodrug nanoparticles. They exhibited excellent pH and reduction of dual-triggered drug release, with cumulative drug release of 88.6% within 51 h in the simulated tumor intracellular microenvironment, while the premature drug leakage was only 13.7% in the simulated normal physiological medium. The in vitro experiments revealed the enhanced antitumor efficacy of the C-PK/- SS-Hy-D NPs than the free DOX at a higher dosage of >10 μg/mL.

Activation of soluble polysaccharides with 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) for use in protein-polysaccharide conjugate vaccines and immunological reagents. II. Selective crosslinking of proteins to CDAP-activated polysaccharides

Covalently linking protein to polysaccharides converts the anti-polysaccharide immune response from a T-cell independent response to one which is T-cell dependent. The organic cyanylating reagent 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) (Vaccine 14:190, 1996) has been used to activate polysaccharides, which can then be reacted with spacer reagents or directly with protein. We wished to explore ways in which proteins could be linked to CDAP-activated polysaccharides to conjugate in a more controlled and selective fashion. To this end, we examined the reaction of nucleophilic amino acids with CDAP-activated polysaccharides under basic and acidic conditions. We found that lysine, cysteine and histidine but not methionine, serine or tyrosine conjugated to CDAP-activated dextran. We also examined the reaction of various spacer reagents with CDAP-activated dextran as a function of pH. The addition of hexanediamine was highly pH dependent and maximal at pH 9.3. In contrast, the addition of adipic dihydrazide, which has a pKa of ca 2.5 was essentially independent of pH. By performing the conjugation reaction at pH 5, we were able to selectively couple hydrazides even in the presence of high concentrations of amines. Proteins derivatized with limited numbers of hydrazides could be conjugated to CDAP-activated polysaccharides at pH5, where the native protein was not reactive. Proteins could be derivatized with hydrazides on carboxyls using adipic dihydrazide and a water soluble carbodiimide or on amines using a mild two-step reaction. Tetanus toxoid-pneumococcal type 14 conjugates produced by coupling hydrazide-derivatized tetanus toxoid under acidic conditions induced anti-polysaccharide antibodies at titers comparable to that stimulated by conjugates produced using a basic coupling pH. Our data suggest that crosslinking was occurring only with the limited number of hydrazides on the protein and that we achieved limited and selective crosslinking between the protein and CDAP-activated polysaccharide. This work also demonstrates that CDAP-mediated conjugation to polysaccharides can be applied even to very pH sensitive proteins and polysaccharides.

Improved method for converting an unmodified peptide to an energy-transfer substrate for a proteinase

This paper describes refinements to a method for generating energy-transfer-based fluorogenic substrates for proteinases [Geoghegan, K. F., et al. (1993) Bioconjugate Chem. 4, 537-544]. An unprotected peptide is taken as starting material and coupled by three steps of chemistry to two dyes that form an energy donor-energy acceptor pair. The donor group is Lucifer Yellow CH (lambda cx 420-430 nm, lambda em 530 nm), and the acceptor is 5-carboxytetramethylrhodamine, a strong chromophore that effectively quenches Lucifer Yellow fluorescence in intact substrates. Periodate oxidation of N-terminal Ser gives the peptide an alpha-N-glyoxylyl moiety, OHC-CO-peptide, which is allowed to react with Lucifer Yellow CH (a carbohydrazide derivative) to attach the dye through an adequately stable hydrazone bond. Derivatization is completed by allowing the succinimidyl ester of 5-carboxytetramethylrhodamine to react with the epsilon-amino group of a single Lys placed near the C terminus. In the revised method, the first two steps of chemistry are completed sequentially in a single vessel to eliminate the labor and losses associated with isolating the alpha-N-glyoxylyl intermediate. In addition, peptides taken as starting material for this method are now designed according to the scheme (Ser)-(cleavable sequence)-(Lys-Arg). The addition of Arg at the C terminus promotes aqueous solubility of the final substrate without complicating the chemistry; multiple Arg might also be used, through this has not been tried here. It has also been found that relative reversed-phase HPLC retention of Lucifer Yellow/5-carboxytetramethylrhodamine substrates is predictable on the basis of the hydrophobicity of the original peptide. Substrates prepared from peptides with Bull and Breese indices of > 200 cal/ mol are readily separated from residual dye in the second and final chromatographic step of the synthesis, simplifying the only moderately difficult step in the preparation. As with all fluorogenic substrates, assays using these substrates are subject to internal filtering effects that can lead to serious error. This problem becomes significant at a range of substrate concentration that is predictable when the size and geometry of the fluorescence cell are taken into account. The reasoning applied in determining this range is applicable to substrates constructed with any donor-acceptor pair.

Toxin-targeted design for anticancer therapy. I: Synthesis and biological evaluation of new thioimidate heterobifunctional reagents

In an effort to obtain a more potent and specific immunotoxin for cancer therapy, we designed a series of heterobifunctional linkers characterized by a thioimidate group linked to a S-acetyl thiol (4, 5) or substituted aryldithio group (6-10). These ligands were synthesized by a Pinner-type process from the corresponding nitrile derivatives obtained by thiol-disulphide exchange reaction, reaction with substituted benzene-sulphenyl chloride, or other known procedures. To check the reagent of choice for immunoconjugate preparation, we studied thioldisulphide exchange kinetics between the intermediate nitrile derivatives and cysteine. Among the tested aryldithio derivatives (6-10), we selected ethyl 3-(4-carboxamido-phenyldithio)propionthioimidate (CDPT, 9) for further studies. By analyzing the rate of incorporation of the linkers 4, 5, and 9 in a model immunoglobulin G protein, we found similar results with CDPT 9 and ethyl S-acetyl 3-mercaptopropionthioimidate ester hydrochloride (AMPT, 5) because both reagents showed a linear correlation between the number of introduced thiol groups and factors such as time and protein and reagent concentrations. Comparison of the two acetylthio-derivative ligands 4 and 5 showed that AMPT 5 was more stable toward deacetylation than ethyl S-acetyl 2-mercaptopropionthioimidate ester hydrochloride (AMAT, 4). By comparing the kinetic and biological parameters of seven new thioimidate linkers, we found that two of these (CDPT and AMPT) could be superior ligands for protein-protein conjugation. They offer advantages over the commercially available compounds, such as minimal perturbation of the protein structure, controlled reactivity, and good stability.