Design of self-immolative linkers for tumour-activated prodrug therapy.

The main drawback of most cancer chemotherapy is its relatively low ability to target tumour cells versus normal cells. As a consequence, chemotherapy is usually connected with severe side effects due to the toxicity of traditional cytostatic agents towards normal tissues. A few years ago, the site-specific activation of non-toxic prodrugs in tumours has been proposed in order to enhance the selectivity for the killing of cancer cells. Within this framework, most of the prodrugs that have been designed were three part compounds comprising trigger, linker and effector units. The main function of the linker is to release the effector unit after selective trigger activation via a spontaneous chemical breakdown. However, its structure also affects significantly many prodrug properties such as stability, pharmacokinetic, organ distribution, bioavailability or trigger activation. This review, focussed on the linker unit, is an update of our previous article published in 2002. It deals with recent advances in the design of prodrug linkers including new delivery systems such as elongated linkers or self-immolative dendrimers.

Design of selectively activated anticancer prodrugs: elimination and cyclization strategies.

Cancer chemotherapy is associated with severe side effects which may be reduced by selective liberation, at the tumour site, of a cytotoxic agent from a non-toxic prodrug. Several strategies are used to achieve the required selective activation: with enzymes which are present in higher concentration in, or close, to the tumour (beta-glucuronidase, plasmin), with enzymes covalently linked to a macromolecular carrier able to recognize antigen positive cancer cells (ADEPT: Antibody Directed Enzyme Prodrug Therapy) or with reductive processes which are favoured in an hypoxic environment. Most of the prodrugs include a linker (or spacer) between the trigger and the drug (or effector). The design of such linkers is crucial in order to achieve a fast drug liberation under physiological conditions. The linker groups may be classified in two categories based on elimination or cyclization processes. The advantages and the limitations of each strategy are discussed.

2-Nitro and 4-nitro-quinone-methides are not irreversible inhibitors of bovine beta-glucuronidase.

4-Benzylamino-(and 4-chloromethyl)-2-nitro-beta-D-glucuronides (4, 10) and their 2-substituted-4-nitro regioisomers (7, 13) were prepared by glycosidation of the 3-nitro-4-hydroxy- and the 2-hydroxy-5-nitro-benzylic alcohol, respectively, with a glucuronyl donor. Carbonate activation followed by reaction with benzylamine or methanesulfonyl chloride afforded, after complete deprotection, the target molecules 4, 7, 10 and 13. These compounds have been synthesized to determine whether these molecules are (or not) glucuronidase inhibitors. After incubation with bovine liver beta-glucuronidase, none of the cleavage products (the titled quinone-methides) showed to be irreversible inhibitors of this enzyme.

New cytotoxic analogues of annonaceous acetogenins.

A series of new acetogenin analogues incorporating a central catechol moiety instead of the tetrahydrofuran ring(s) have been prepared and tested against L1210 leukemia cells. Although less potent than bullatacinone, which has the same terminal lactone, these compounds display interesting cell cycle effects.

Synthesis and cytotoxic activity of a glucuronylated prodrug of nornitrogen mustard.

A new glucuronylated prodrug of nornitrogen mustard, incorporating the same spacer group as the doxorubicin prodrug HMR 1826, has been prepared. Upon exposure to E. coli beta-glucuronidase, fast hydrolysis occurs but a lower cytotoxicity against LoVo cancer cells is observed compared to the nornitrogen mustard alone. This is explained by cyclization of the intermediate carbamic acid to the inactive chloroethyl oxazolidinone.

Synthesis and cytotoxic activity of acetogenin analogues.

A set of 16 new simplified analogues of acetogenins has been designed based on: (i) the replacement of the bis THF moiety of these natural products by an ethylene glycol bis ether unit; (ii) the introduction of different lipophilic side chains (alkyl, aryl, dialkylamino, O-cholesteryl); (iii) the presence of the same terminal isolactone. In vitro cytotoxic activity against L1210 leukemia is reported.

Prodrugs of anthracyclines for use in antibody-directed enzyme prodrug therapy.

A series of new prodrugs of daunorubicin and doxorubicin which are candidates for antibody-directed enzyme prodrug therapy (ADEPT) is reported. These compounds (25a,b,c and 32a,b,c) have been designed to generate cytotoxic drugs after activation with beta-glucuronidase. As expected, recovery of the active drug was observed after enzymatic cleavage by Escherichia coli beta-glucuronidase as well as by a fusion protein which has been obtained from human beta-glucuronidase and humanized CEA-specific binding region. The six prodrugs are highly stable and are more than 100-fold less cytotoxic than doxorubicin against murine L1210 cell lines. The ortho-substituted phenyl carbamates 25a,b,c are better substrates for beta-glucuronidase than the corresponding para-substituted analogues. After taking into account additional factors such as stability in plasma and kinetics of enzymatic cleavage, we selected the o-nitro prodrug 25c for clinical trials.

Elucidation of the mechanism enabling tumor selective prodrug monotherapy.

Elucidation of the mechanism enabling tumor selective PMT in vivo with appropriate glucuronyl-spacer-doxorubicin prodrugs, such as HMR 1826, is important for the design of clinical studies, as well as for the development of more selective drugs. Enzyme histochemistry, immunohistochemistry, and the terminal deoxytransferase technique were applied using human cryopreserved cancer tissues, normal human, monkey, and mouse tissues, and human tumor xenografts to examine mechanisms underlying the selectivity of successful PMT with HMR 1826. It could unambiguously be shown by enzyme histochemistry that necrotic areas in human cancers are the sites in which lysosomal beta-glucuronidase is liberated extracellularly in high local concentrations. The cells responsible for the liberation of the enzyme are mainly acute and chronic inflammatory cells, as shown by IHC. Furthermore, it could be demonstrated that beta-glucuronidase liberated in necrotic areas of tumors can activate HMR 1826, resulting in increased doxorubicin deposition in human tumor xenografts or in human lung cancers subjected to extracorporal perfusion, compared to chemotherapy with doxorubicin. Additionally, the doxorubicin load to normal tissues was significantly reduced compared to chemotherapy with doxorubicin. Surprisingly, the increased doxorubicin deposition in tumors also resulted in strong antitumor effects also in cancers resistant to maximum tolerated doses of systemic doxorubicin. Finally, toxicity studies in mice and monkeys revealed an excellent tolerability of HMR 1826, up to a dose of 3 g/m2 (monkeys). These data suggest that HMR 1826 is a promising candidate for clinical development.

A new spacer group derived from arylmalonaldehydes for glucuronylated prodrugs.

A new glucuronylated prodrug of doxorubicin, potentially useful for ADEPT or PMT cancer chemotherapy, has been prepared from 4-methyl phenyl malonaldehyde. The enol ether spacer, linked via a carbamate to the 3'-amino group of doxorubicin is rapidly cleaved after beta-glucuronidase (E coli) catalyzed hydrolysis at pH 7.2 and 37 degrees C.

Prodrugs of anthracycline antibiotics suited for tumor-specific activation.

The two novel prodrugs 4 and 11 have been prepared from tetra-O-acetyl-D-galactopyranose and doxorubicin in three and six steps, respectively. Their low cytotoxicity, high stability in plasma and, in the case of 11, efficient hydrolysis in the presence of alpha-galactosidase, fulfill preliminary conditions for their use in combination with monoclonal antibody-enzyme conjugates.

Prodrugs of anthracyclines for chemotherapy via enzyme-monoclonal antibody conjugates.

New prodrugs of daunorubicin, 1c, 1e and 2c, including a galactopyranosyl residue linked to the N-3' of the daunosaminyl moiety through substituted o- or p-benzyloxycarbonyl groups were synthesized. Their low cytotoxicity and high stability in plasma fulfil the conditions for antibody-directed enzyme prodrug therapy (ADEPT). Enzymatic hydrolysis using alpha-D-galactosidase gives rise to daunorubicin by subsequent self-elimination of the spacers. However, elimination clearly depends on the aromatic substitution pattern, as demonstrated especially by comparison with non-substituted analogues.