Anticancer Agents, 15. Squaric Acid Diethyl Ester: A New Coupling Reagent for the Formation of Drug Biopolymer Conjugates. Synthesis of Squaric Acid Ester Amides and Diamides

Abstract

Reaction of squaric acid diethyl ester (1) with a slight excess of a primary or secondary amine 2 in ethanol, dichloromethane or aqueous buffer (pH 3 ) a t 20°C for 0.3-12 h gives the squaric acid amide esters 3 in mostly excellent yields. Treatment of 3 with amines 2 or 4 in organic solvents in the presence of triethylamine or in aqueous buffer (pH 9) leads to the corresponding symmetrical and unsymmetrical squaric acid diamides 5, respectively. The reaction can be followed by UV spectroscopy.There is a great need for the development of new mild coupling procedures to immobilize enzymes or to bind pharmacologically active molecules to polymers'). Thus, the use of biopolymer drug conjugates is a promising approach to the treatment and diagnosis of many deseases3! Conjugates of radioactive compounds or dyes with monoclonal antibodies which bind with some specifity to tumor-associated surface antigens are successfully used in the diagnosis of cancer4); indeed, a number of monoclonal antibodies directed against cell surface antigens on human tumors such as breast and colon carcinomas, melanomas and gliomas have already been prepared using the hybridoma technique.The advantage of applying conjugates lies in the selective delivery to the target site and also in the possible protection of drugs against fast cnzyrnatic degration and excretion, thereby leading to a higher drug concentration in the tumor. However, treatment of cancer with monoclonal antibody-drug conjugates has not been successful so far, in part due to the low accessibility of malignant cells in the interior parts of tumors; thus, the drug antibody complex is cytotoxic only to those cells that bind the antibody. In our concept for the development of new anticancer drugs with increased tumor selectivity and lower general toxicity we also use monoclonal antibodies for targeting, but in contrast to known approaches our procedure aims at the liberation of the anticancer drug from the monoclonal antibody within the tumor tissue via tumor-selective, proton-mediated activation of a prodrug".Several methods have been developed for the coupling of small molecules to proteins and other bi~polymers~*~). Many of these proccdures requirc harsh conditions, which may cause a loss of the 'go N H R T ( 2 ) -'go N H R 3 z ( 4 ) _ O x 01 3 6 rt = room temperature affinity and specificity of the monoclonal antibody employed. Moreover, most of these methods do not allow an effective determination of the coupling rate. Finally, within the development of our more selective anticancer agents slightly basic conditions are required for coupling because of the acid lability of these compounds.In this paper we describe the coupling of two amines using squaric acid diethyl ester') by sequential formation of the squaric acid amide ester and squaric acid diamides. The reaction can be performed in organic solvents and in water with primary and secondary amines under mild conditions. Treatment of squaric acid diethyl ester (1) with a slight excess of a primary or a secondary amine 2 in ...

A New Series of Glycopeptide Antibiotics Incorporating a Squaric Acid Moiety

“…In a recent work we have successfully applied squaric acid esters, introduced by Tietze et al [8], for the chemical modification of anthracycline glycoside antibiotics [9,10]. It is known that squaric esters (7) selectively react with a small excess of primary or secondary amines in aqueous and/or alcoholic media even in the presence of alcoholic and phenolic hydroxyl, and also carboxyl groups.…”

“…It is known that squaric esters (7) selectively react with a small excess of primary or secondary amines in aqueous and/or alcoholic media even in the presence of alcoholic and phenolic hydroxyl, and also carboxyl groups. In neutral media the product is always the squaric acid amide ester (8), whereas under more alkaline conditions (pH Ͼ7) the corresponding symmetric (9) or asymmetric diamide (10) are produced (Scheme 3).…”

“…The most intense peak in the spectrum at m/zϭ1676.3 is formed by loss of 144 Da. The mass of both R 2 and R 3 (for abbreviations see Scheme 1 and 2) is 144 but the consecutive loss of 305 Da resulting a peak at m/zϭ1371.1 clearly shows that the former is due to 572 Scheme 3 The squarate linker method [8]. the loss of R 3 and the latter is that of the disaccharide chain containing R 2 , respectively.…”

A New Class of Semisynthetic Anthracycline Glycoside Antibiotics Incorporating a Squaric Acid Moiety

“…It is well-known that the various squaric acid derivatives possess low solubility in water [17]. However, introduction of carbohydrate moieties would enhance the watersolubility and decrease the disadvantageous toxic effects.…”

“…In a recent work, Sztaricskai and coworkers have successfully applied squaric acid esters, introduced to organic chemistry by Tietze et al [17], for the chemical synthesis [18] of the squaric acid amide esters (7ϳ9) and the corresponding covalent dimers (10ϳ13) of the anthracycline glycoside antibiotics 1ϳ3 (Fig. 2).…”