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.

Aggressive behavior induced by the steroid sulfatase inhibitor COUMATE and by DHEAS in CBA/H mice

The steroid sulfatase enzyme (STS) regulates the formation of dehydroepiandrosterone (DHEA) from dehydroepiandrosterone-sulfate (DHEAS). DHEAS is a well-known negative allosteric modulator of the GABA(A) receptor-gated chloride channels. It is classified as an excitatory neurosteroid. The implication of GABA(A) receptor activity in aggressive behavior in rodents is well-documented. In addition a genetic correlation between STS level in the liver and aggressive behavior across 12 strains of mice suggest that STS activity could be involved in aggression in mice. We assessed herein whether COUMATE (an STS inhibitor) and DHEAS modulate aggression in CBA/H mice. We hypothesized that inhibiting STS activity in vivo followed by DHEAS injections which increase the level of sulfated steroid that cross the blood-brain barrier and then modulate neurotransmitter receptors could modify the attack behavior in mice. COUMATE (10 mg/kg) was administrated p.o. alone or in combination with the neurosteroid DHEAS (0-50 mg/kg) i.p. Animals were thereafter tested for aggression. A single dose of COUMATE significantly inhibited STS activity both in the brain (70.57%) and in the liver (87%) 24 h following administration. Behavioral tests showed that the inhibitor and DHEAS enhanced aggressive behavior when animals were simultaneously subjected to both molecules. These results confirm the correlation between aggressive behavior and STS concentration in mice. In addition, we confirm that the steroid metabolism can modulate the behavior in rodents.

Synthesis of indomethacin analogues for evaluation as modulators of MRP activity

Synthesis of a range of indomethacin analogues, required for investigation in combination toxicity assays, bearing both N-benzyl and N-benzoyl groups, is described.

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.

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.