Neuroblastoma directed therapy by a rational prodrug design of etoposide as a substrate for tyrosine hydroxylase

Turn on chemiluminescence-based probes for monitoring tyrosinase activity in conjunction with biological thiols

We report a chemiluminescent probe (CLPT1) that permits the paired detection of tyrosinase (Tyr) and biological thiols. Tyr only leads to a poor chemiluminescence response, a finding ascribed to the formation of a stable o-benzoquinone intermediate. The addition of glutathione (GSH), or ascorbate to the o-benzoquinone intermediate results in thiol conjugation or reduction to this intermediate, respectively. This produces a strong chemiluminescence response. Thiol co-dependence was demonstrated in live cells using the cell permeable analogue, CLPT3. The present chemiluminescence-based strategy allows the concurrent detection of tyrosinase activity and biological thiols.

Modifications of human carboxylesterase for improved prodrug activation

BACKGROUND

Carboxylesterases (CEs) are ubiquitous enzymes responsible for the hydrolysis of numerous clinically useful drugs. As ester moieties are frequently included in molecules to improve their water solubility and bioavailability, de facto they become substrates for CEs.

OBJECTIVE

In this review, we describe the properties of human CEs with regard to their ability to activate anticancer prodrugs and demonstrate how structure-based design can be used to modulate substrate specificity and to increase efficiency of hydrolysis.

METHODS

A specific example using CPT-11 and a human liver CE is discussed. However, these techniques can be applied to other enzymes and their associated prodrugs.

RESULTS

Structure-guided mutagenesis of CEs can be employed to alter substrate specificity and generate novel enzymes that are efficacious at anticancer prodrug activation.

Solution-phase parallel synthesis of carbamates as gamma-secretase inhibitors

A novel methodology for parallel liquid-phase synthesis of carbamates suitable for the preparation of sterically hindered molecules is disclosed. The alcohols are converted to 4-nitrophenylcarbonates, followed by the reaction with amines. Side product 4-nitrophenol and the unreacted excess amines are scavenged by appropriately chosen cleanup resins, selected among Amberlyst A26 (hydroxide form) and macroporous sulfonic acid (MP-TsOH) or polystyrene isocyanate (PS-NCO) and polystyrene benzaldehyde (PS-PhCHO) resins. As a part of a medicinal chemistry program directed toward finding gamma-secretase inhibitors as prospective drug candidates for Alzheimer's disease, a 6 x 24 library of carbamates was prepared. Out of 144 library members, 133 had a purity for the targeted compound of 80% or better. The prepared compounds were assessed in the gamma-secretase inhibition assay and demonstrated activity with IC 50 values in the range from 1 microM to 5 nM, with the activity of 7 compounds being better than 10 nM.

Suicide genes for cancer therapy

The principle of using suicide genes for gene directed enzyme prodrug therapy (GDEPT) of cancer has gained increasing significance during the 20 years since its inception. The astute application of suitable GDEPT systems should permit tumour ablation in the absence of off-target toxicity commonly associated with classical chemotherapy, a hypothesis which is supported by encouraging results in a multitude of pre-clinical animal models. This review provides a clear explanation of the rationale behind the GDEPT principle, outlining the advantages and limitations of different GDEPT strategies with respect to the roles of the bystander effect, the immune system and the selectivity of the activated prodrug in contributing to their therapeutic efficacy. An in-depth analysis of the most widely used suicide gene/prodrug combinations is presented, including details of the latest advances in enzyme and prodrug optimisation and results from the most recent clinical trials.

Development of an etoposide prodrug for dual prodrug-enzyme antitumor therapy

Enzyme-prodrug approaches to cancer therapy, theoretically, have the potential to mediate tumor-selective cytotoxicity. However, even if tumor-specific prodrug activation is achieved, enzyme-prodrug systems investigated thus far comprised a single enzyme and a specific prodrug. Although targeted, such systems constitute single-agent therapy, which may be ineffective and/or may promote development of drug resistance. Therefore, a goal of our laboratories was to design and characterize a novel dipiperidinyl derivative of etoposide [1,4'-dipiperidine-1'-carboxylate-etoposide (dp-VP16)] that would act as a prodrug. We envisioned that dp-VP16 would be converted to the active chemotherapeutic agent VP-16 by the same rabbit carboxylesterase (rCE) that we have previously shown to efficiently activate the prodrug irinotecan (CPT-11). This dp-VP16 prodrug might then be used in combination with CPT-11, with both drugs activated by a single enzyme. We evaluated the ability of pure rCE and two human carboxylesterases, hCE1 and hiCE (hCE2), to activate dp-VP16 in vitro, and in neuroblastoma cell lines designed to express/overexpress each enzyme. In SK-N-AS neuroblastoma cell transfectants, expression of rCE or hiCE decreased the IC50 of dp-VP16 as a single agent by 8.3- and 3.4-fold, respectively, in growth inhibition assays. Purified hCE1 did not metabolize dp-VP16 in vitro and did not affect its IC50 in intact cells. The combination indices of sequential exposure to CPT-11 followed by dp-VP16 ranged from approximately 0.4 to 0.6, suggesting that this combination produced greater-than-additive cytotoxicity in neuroblastoma cells expressing rCE. These data provide proof-of-principle that enzyme-prodrug therapy approaches comprised of prodrugs with complementary mechanisms of cytotoxicity that are activated by a single enzyme can be developed.