The antitumour activity of the prodrug N-L-leucyl-doxorubicin and its parent compound doxorubicin in human tumour xenografts

Turning antibodies off and on again using a covalently tethered blocking peptide

In their natural form, antibodies are always in an "on-state" and are capable of binding to their targets. This leads to undesirable interactions in a wide range of therapeutic, analytical, and synthetic applications. Modulating binding kinetics of antibodies to turn them from an "off-state" to an "on-state" with temporal and spatial control can address this. Here we demonstrate a method to modulate binding activity of antibodies in a predictable and reproducible way. We designed a blocking construct that uses both covalent and non-covalent interactions with the antibody. The construct consisted of a Protein L protein attached to a flexible linker ending in a blocking-peptide designed to interact with the antibody binding site. A mutant Protein L was developed to enable photo-triggered covalent crosslinking to the antibody at a specific location. The covalent bond anchored the linker and blocking peptide to the antibody light chain keeping the blocking peptide close to the antibody binding site. This effectively put the antibody into an "off-state". We demonstrate that protease-cleavable and photocleavable moieties in the tether enable controlled antibody activation to the "on-state" for anti-FLAG and cetuximab antibodies. Protein L can bind a range of antibodies used therapeutically and in research for wide applicability.

Protease-activated prodrugs: strategies, challenges, and future directions

Proteases play critical roles in virtually all biological processes, including proliferation, cell death and survival, protein turnover, and migration. However, when dysregulated, these enzymes contribute to the progression of multiple diseases, with cancer, neurodegenerative disorders, inflammation, and blood disorders being the most prominent examples. For a long time, disease-associated proteases have been used for the activation of various prodrugs due to their well-characterized catalytic activity and ability to selectively cleave only those substrates that strictly correspond with their active site architecture. To date, versatile peptide sequences that are cleaved by proteases in a site-specific manner have been utilized as bioactive linkers for the targeted delivery of multiple types of cargo, including fluorescent dyes, photosensitizers, cytotoxic drugs, antibiotics, and pro-antibodies. This platform is highly adaptive, as multiple protease-labile conjugates have already been developed, some of which are currently in clinical use for cancer treatment. In this review, recent advancements in the development of novel protease-cleavable linkers for selective drug delivery are described. Moreover, the current limitations regarding the selectivity of linkers are discussed, and the future perspectives that rely on the application of unnatural amino acids for the development of highly selective peptide linkers are also presented.

Antitumor agents 7. Synthesis, antiproliferative activity and molecular modeling of new l-lysine-conjugated pyridophenoxazinones as potent DNA-binding ligands and topoisomerase IIα inhibitors

A series of l-lysine-conjugated pyridophenoxazinones 2-5 and 2'-5' were designed and synthesized for developing compounds with multimodal anticancer potentialities. All compounds inhibited the proliferation of a panel of human liquid and solid neoplastic cell lines. 2 and 5 were the most active compounds with IC₅₀ values in the submicromolar range. UV-vis, ¹H NMR, unwinding, and docking experiments demonstrated that they intercalate between the middle 5'-GC-3' base pairs with the carboxamide side chain lying into major groove. Charge-transfer contribution to the complex stability, evaluated by ab initio calculations, was found to correlate with cytotoxicity. Relaxation and cleavage assays showed that 2 and 5 selectively target Topo IIα over Topo IIβ and stimulate the formation of covalent Topo II-DNA complexes, functioning as poisons. Moreover, compound 5 induced DNA damage and arrested MCF-7 cells at the G2/M phase. Altogether, the work provides interesting structure-activity relationships in the pyridophenoxazinone-l-lysine conjugate series and identifies 5 as a promising candidate for further in vivo evaluation.

A sensitive liquid chromatography/electrospray tandem mass spectroscopy method for simultaneous quantification of a disulfide bond doxorubicin conjugation prodrug and activated doxorubicin: Application to cellular pharmacokinetic and catabolism studies

In recent years, drug conjugates as a prodrug strategy have been widely studied, especially combined with nanotechnology. Disulfide-linked doxorubicin drug-drug conjugate (DOX-S-S-DOX) nanoparticles, have recently been developed as a doxorubicin prodrug nanoparticles with greater anticancer activity and less toxicity than doxorubicin in vivo, while its intracellular kinetics and metabolism is unclear which may provide us with a deeper understanding of its pharmacological mechanism and antitumor effect. Hence, in this study, a rapid and sensitive ultra high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed to detect doxorubicin (DOX) activated from DOX-S-S-DOX, as well as the prodrug itself in human breast cancer tumor cells (MCF-7). Sample preparation involved acetonitrile precipitation to extract the analytes simultaneously and bath sonication to remove intercalated DOX from DNA. The calibration range was 3-60ng/mL for DOX and 20-400ng/mL for DOX-S-S-DOX with the correlation coefficients (r²)≥0.99, using daunorubicin as internal standard (IS). The inter- and intra-assay precision (relative standard deviation, RSD%) of quality control samples was in the acceptable range (<15%) and relative error (RE%) for accuracy was between -5.35 and 9.18% for all analytes. Recovery (59.28-69.53% for DOX-S-S-DOX and 99.13-100.10% for DOX) and matrix effect (99.69-111.19%) was consistent, precise, and reproducible at different quality control levels in accordance with FDA guidance. Stability studies showed that DOX-S-S-DOX was unstable both during the bench-top and long-term storage, while the stability during sample preparation and LC-MS runtime was suitable for all the analytes. Hence, the samples should be prepared as soon as possible at the time point to prevent the catabolism of DOX-S-S-DOX. The assay was successfully used in the cellular metabolism and pharmacokinetics study of DOX-S-S-DOX and it may give a clue to explore analytical methods of other prodrug forms of DOX.

Drug Delivery Nanoparticles with Locally Tunable Toxicity Made Entirely from a Light-Activatable Prodrug of Doxorubicin

PURPOSE

A major challenge facing nanoparticle-based delivery of chemotherapy agents is the natural and unavoidable accumulation of these particles in healthy tissue resulting in local toxicity and dose-limiting side effects. To address this issue, we have designed and characterized a new prodrug nanoparticle with controllable toxicity allowing a locally-delivered light trigger to convert the payload of the particle from a low to a high toxicity state.

METHODS

The nanoparticles are created entirely from light-activatable prodrug molecules using a nanoprecipitation process. The prodrug is a conjugate of doxorubicin and photocleavable biotin (DOX-PCB).

RESULTS

These DOX-PCB nanoparticles are 30 times less toxic to cells than doxorubicin, but can be activated to release pure therapeutic doxorubicin when exposed to 365 nm light. These nanoparticles have an average diameter of around 100 nm and achieve the maximum possible prodrug loading capacity since no support structure or coating is required to prevent loss of prodrug from the nanoparticle.

CONCLUSIONS

These light activatable nanoparticles demonstrate tunable toxicity and can be used to facilitate future therapy development whereby light delivered specifically to the tumor tissue would locally convert the nanoparticles to doxorubicin while leaving nanoparticles accumulated in healthy tissue in the less toxic prodrug form.

Legumain-cleavable 4-arm poly(ethylene glycol)-doxorubicin conjugate for tumor specific delivery and release

Traditional chemotherapy strategy exists undesirable toxic side-effects to normal tissues due to the low selectively to cancer cells of micromolecule cytotoxic drugs. One considered method to realizing the targeted delivery and increasing the specificity to tumor tissues of the cytotoxic drug is to transporting and discharging it through an environment-sensitive mechanism. In this study, a novel enzyme-sensitive polymer-doxorubicin conjugate was designed to delivery chemotherapeutic drug in a tumor-specific behavior and selectively activated in tumor tissue. Briefly, doxorubicin (DOX) was conjugated to carboxyl-terminated 4-arm poly(ethylene glycol) through a tetrapeptide linker, alanine-alanine-asparagine-leucine (AANL), which was one of the substrates of legumain, an asparaginyl endopeptidase that was found presented in plants, mammals and also highly expressed in human tumor tissues. Hereinafter, the polymer-DOX conjugate was termed as 4-arm PEG-AANL-DOX. Dynamic laser scattering (DLS) and transmission electron microscopy (TEM) measurements indicated that the 4-arm PEG-AANL-DOX could self-assemble into micelles in aqueous solution. Drug release and in vitro cytotoxicity studies revealed that the 4-arm PEG-AANL-DOX could be cleaved by legumain. Ex vivo DOX fluorescence imaging measurements demonstrated that the 4-arm PEG-AANL-DOX had an improved tumor-targeting delivery as compared with the free DOX·HCl. In vivo studies on nude mice bearing MDA-MB-435 tumors revealed that the 4-arm PEG-AANL-DOX had a comparable anticancer efficacy with the free DOX·HCl but without DOX-related toxicities to normal tissues as measured by body weight change and histological assessments, indicating that the 4-arm PEG-AANL-DOX had an improved therapeutic index for cancer therapy.

STATEMENT OF SIGNIFICANCE

Herein we describe the construction of a novel tumor environment-sensitive delivery system through the instruction of a legumain-cleavable linkage to a polymer-DOX conjugate (4-arm PEG-AANL-DOX). This particular design strategy allows for polymer-DOX conjugates to be delivered in a tumor-specific manner and selectively activable in tumor microenvironment so that it can combine the advantages of tumor-specific delivery and tumor intracellular microenvironment-triggered release systems.

Monitoring subcellular biotransformation of N-L-leucyldoxorubicin by micellar electrokinetic capillary chromatography coupled to laser-induced fluorescence detection

Development of prodrugs is a promising alternative to address cytotoxicity and nonspecificity of common anticancer agents. N-L-leucyldoxorubicin (LeuDox) is a prodrug that is biotransformed to the anticancer drug doxorubicin (Dox) in the extracellular space; however, its biotransformation may also occur intracellularly in endocytic organelles. Such organelle-specific biotransformation is yet to be determined. In this study, magnetically enriched endocytic organelle fractions from human uterine sarcoma cells were treated with LeuDox. Micellar electrokinetic chromatography with laser-induced fluorescence detection (MEKC-LIF) was used to determine that 10% of LeuDox was biotransformed to Dox, accounting for ~43% of the biotransformation occurring in the post-nuclear fraction. This finding suggests that endocytic organelles also participate in the intracellular biotransformation of LeuDox to Dox.

Development of a tumor-specific photoactivatable doxorubicin prodrug

This is a retrospective highlight on the publication by Ibsen and coworkers: Localized In Vivo Activation of a Photoactivatable Doxorubicin Prodrug in Deep Tumor Tissue, which appeared in a preceding issue of Photochem. Photobiol. (2013, 89:698-708). The authors describe the synthesis and properties of a novel doxorubicin (DOX) prodrug, DOX-PCB, which contains a photocleavable linker group. Systemic administration of the prodrug to a tumor-bearing animal followed by LED/fiber optic 365 nm light delivery allowed active DOX to be released site specifically in the tumor area. This elegant and timely study provides compelling evidence that photocleavable DOX-PCB can eliminate many of the toxic side effects of DOX that have plagued clinical use of this highly effective antitumor drug for many years.

Extraction protocol and mass spectrometry method for quantification of doxorubicin released locally from prodrugs in tumor tissue

The localized conversion of inactive doxorubicin prodrug chemotherapeutics to pharmacalogically active forms is difficult to quantify in mouse tumor models because it occurs only in small regions of tissue. The tumor tissue extraction protocol and LC-MS/MS analysis method described here were optimized to obtain a detection limit of 7.8 pg for the activated doxorubicin and 0.36 ng for the doxorubicin prodrug. This method can be useful for determining the biodistribution and activation efficiency for many different doxorubicin prodrugs. It can also be used for quantification of doxorubicin from tumor models that have poor vascularization resulting in low tissue accumulation.

Microbubble-mediated ultrasound therapy: a review of its potential in cancer treatment

The inherently toxic nature of chemotherapy drugs is essential for them to kill cancer cells but is also the source of the detrimental side effects experienced by patients. One strategy to reduce these side effects is to limit the healthy tissue exposure by encapsulating the drugs in a vehicle that demonstrates a very low leak rate in circulation while simultaneously having the potential for rapid release once inside the tumor. Designing a vehicle with these two opposing properties is the major challenge in the field of drug delivery. A triggering event is required to change the vehicle from its stable circulating state to its unstable release state. A unique mechanical actuation type trigger is possible by harnessing the size changes that occur when microbubbles interact with ultrasound. These mechanical actuations can burst liposomes and cell membranes alike allowing for rapid drug release and facilitating delivery into nearby cells. The tight focusing ability of the ultrasound to just a few cubic millimeters allows for precise control over the tissue location where the microbubbles destabilize the vehicles. This allows the ultrasound to highlight the tumor tissue and cause rapid drug release from any carrier present. Different vehicle designs have been demonstrated from carrying drug on just the surface of the microbubble itself to encapsulating the microbubble along with the drug within a liposome. In the future, nanoparticles may extend the circulation half-life of these ultrasound triggerable drug-delivery vehicles by acting as nucleation sites of ultrasound-induced mechanical actuation. In addition to the drug delivery capability, the microbubble size changes can also be used to create imaging contrast agents that could allow the internal chemical environment of a tumor to be studied to help improve the diagnosis and detection of cancer. The ability to attain truly tumor-specific release from circulating drug-delivery vehicles is an exciting future prospect to reduce chemotherapy side effects while increasing drug effectiveness.

Localized in vivo activation of a photoactivatable doxorubicin prodrug in deep tumor tissue

Sparing sensitive healthy tissue from chemotherapy exposure is a critical challenge in the treatment of cancer. The work described here demonstrates the localized in vivo photoactivation of a new chemotherapy prodrug of doxorubicin (DOX). The DOX prodrug (DOX-PCB) was 200 times less toxic than DOX and was designed to release pure DOX when exposed to 365 nm light. This wavelength was chosen because it had good tissue penetration through a 1 cm diameter tumor, but had very low skin penetration, due to melanin absorption, preventing uncontrolled activation from outside sources. The light was delivered specifically to the tumor tissue using a specialized fiber-optic LED system. Pharmacokinetic studies showed that DOX-PCB had an α circulation half-life of 10 min which was comparable to that of DOX at 20 min. DOX-PCB demonstrated resistance to metabolic cleavage ensuring that exposure to 365 nm light was the main mode of in vivo activation. Tissue extractions from tumors exposed to 365 nm light in vivo showed the presence of DOX-PCB as well as activated DOX. The exposed tumors had six times more DOX concentration than nearby unexposed control tumors. This in vivo proof of concept demonstrates the first preferential activation of a photocleavable prodrug in deep tumor tissue.

The feasibility of enzyme targeted activation for amino acid/dipeptide monoester prodrugs of floxuridine; cathepsin D as a potential targeted enzyme

The improvement of therapeutic efficacy for cancer agents has been a big challenge which includes the increase of tumor selectivity and the reduction of adverse effects at non-tumor sites. In order to achieve those goals, prodrug approaches have been extensively investigated. In this report, the potential activation enzymes for 5′-amino acid/dipeptide monoester floxuridine prodrugs in pancreatic cancer cells were selected and the feasibility of enzyme specific activation of prodrugs was evaluated. All prodrugs exhibited the range of 3.0–105.7 min of half life in Capan-2 cell homogenate with the presence and the absence of selective enzyme inhibitors. 5′-O-L--Phenylalanyl-L-tyrosyl-floxuridine exhibited longer half life only with the presence of pepstatin A. Human cathepsin B and D selectively hydrolized 5′-O-L-phenylalanyl-L-tyrosylfloxuridine and 5′-O-L-phenylalanyl-L-glycylfloxuridine compared to the other tested prodrugs. The wide range of growth inhibitory effect by floxuridine prodrugs in Capan-2 cells was observed due to the different affinities of prodrug promoieties to enyzmes. In conclusion, it is feasible to design prodrugs which are activated by specific enzymes. Cathepsin D might be a good candidate as a target enzyme for prodrug activation and 5′-O-L-phenylalanyl-L-tyrosylfloxuridine may be the best candidate among the tested floxuridine prodrugs.

Protease-activated drug development

In this extensive review, we elucidate the importance of proteases and their role in drug development in various diseases with an emphasis on cancer. First, key proteases are introduced along with their function in disease progression. Next, we link these proteases as targets for the development of prodrugs and provide clinical examples of protease-activatable prodrugs. Finally, we provide significant design considerations needed for the development of the next generation protease-targeted and protease-activatable prodrugs.

A novel Doxorubicin prodrug with controllable photolysis activation for cancer chemotherapy

Purpose

Doxorubicin (DOX) is a very effective anticancer agent. However, in its pure form, its application is limited by significant cardiotoxic side effects. The purpose of this study was to develop a controllably activatable chemotherapy prodrug of DOX created by blocking its free amine group with a biotinylated photocleavable blocking group (PCB).

Methods

An n-hydroxy succunamide protecting group on the PCB allowed selective binding at the DOX active amine group. The PCB included an ortho-nitrophenyl group for photo cleavability and a water-soluble glycol spacer arm ending in a biotin group for enhanced membrane interaction.

Results

This novel DOX-PCB prodrug had a 200-fold decrease in cytotoxicity compared to free DOX and could release active DOX upon exposure to UV light at 350 nm. Unlike DOX, DOX-PCB stayed in the cell cytoplasm, did not enter the nucleus, and did not stain the exposed DNA during mitosis. Human liver microsome incubation with DOX-PCB indicated stability against liver metabolic breakdown.

Conclusions

The development of the DOX-PCB prodrug demonstrates the possibility of using light as a method of prodrug activation in deep internal tissues without relying on inherent physical or biochemical differences between the tumor and healthy tissue for use as the trigger.

Design, synthesis and bioevaluation of novel maleamic amino acid ester conjugates of 3,5-bisarylmethylene-4-piperidones as cytostatic agents

A novel series of maleamic amino acid ester conjugates of 3,5-bisarylmethylene-4-piperidones were prepared to investigate the efficacy of micronutrient conjugation in enhancing cytotoxic potency by improving selectivity and delivery. These compounds, prepared as anticancer agents, were expected to demonstrate enhanced selectivity towards malignant cells through the inhibition of topoisomerase IIalpha via protein thiolation. The cytostatic effects of these compounds were evaluated against three cell lines, namely murine L1210 leukemia cells, human Molt 4/C8 and CEM T-lymphocyte cells. All compounds were found to have greater potency than the reference drug melphalan. Several compounds were found to potently inhibit topoisomerase IIalpha and displayed cytostatic activity in the nanomolar range.

L-histidine-based pH-sensitive anticancer drug carrier micelle: reconstitution and brief evaluation of its systemic toxicity

A doxorubicin (DOX)-carrier micellar system consisting of poly(histidine)(5K)-b-poly(ethylene glycol)(2K) and poly(l-lactic acid)(3K)-b-PEG(2K)-folate has been developed targeting the early endosomal pH and it have been convincingly proved that intracellular high dose strategy using such micelles is effective in overcoming multidrug resistance (MDR) of cancer cells. Due to the low DOX concentrations in the micelle solution obtained by dialysis and the lack of long-term stability of the micelles, stable and lyophilized micelle formulations were the subject of investigation reported here by using excipients of sucrose, PEG or Pluronic. The reconstituted micelle solutions were examined by particle size, pH sensitivity, and cytotoxicity for MDR cells and the results were compared with the non-lyophilized micelles. Among tested excipients, Pluronic F127 (33 wt%) added to the polymer/drug solution prior to dialysis resulted in a reconstituted product stable for a week and presented equivalent benefits as the fresh micelle formulation. The blank micelles did not present any apparent systemic toxicity in mice up to 2400 mg/kg i.v. injection (800 mg/kg day) for 3 days). The brief toxicity of reconstituted DOX loaded micelles was examined by the maximum tolerated dose (MTD), which was approximately 7.5-fold higher than free DOX and guaranteeing further animal toxicity and efficacy study.

Monitoring incorporation, transformation and subcellular distribution of N-l-leucyl-doxorubicin in uterine sarcoma cells using capillary electrophoretic techniques

Previous reports have demonstrated that N-l-leucyl-doxorubicin (LeuDox) is less toxic than its parent drug, Dox, but the underlying causes of this reduced toxicity have yet to be fully elucidated. In this study, the incorporation of LeuDox into (i) the MES-SA human uterine sarcoma cell line and (ii) its Dox resistant counterpart, MES-SA/Dx5 cell line and the subsequent transformation of LeuDox into Dox and its subcellular distribution, were investigated by micellar electrokinetic chromatography with laser-induced fluorescence detection (MEKC-LIF). In both cell lines the cellular uptakes of Dox and LeuDox were similar at equimolar doses, while the percent transformation of LeuDox into Dox in MES-SA/Dx5 cells was about twice as great as its transformation in MES-SA cells, which is beneficial for reaching Dox cytotoxic levels in this resistant cell line. When both cells lines were treated with IC(35) concentrations of either Dox and LeuDox, the intracellular Dox amounts were 6-fold higher in the resistant cell line than in the sensitive cell line, suggesting that other cellular processes play a role in the cytotoxicity of Dox in the resistant cell line. The amounts and ratios of Dox and LeuDox in four subcellular fractions of LeuDox-treated MES-SA/Dx5 cells were also investigated. The highest Dox/LeuDox ratio (i.e. 2.92) was found in the nuclear fraction, followed by the ratio in the low density organelle fraction (i.e. 1.92) that contains lysosomes, organelles in which lysosomal hydrolytic enzymes, capthesins, transform LeuDox into Dox.

Anti-tumor and anti-metastatic effects of gelatin-doxorubicin and PEGylated gelatin-doxorubicin nanoparticles in SCC7 bearing mice

The goal of this study was to develop a systemically non-toxic and stable circulation based passive targeting system for efficient anticancer treatment. Gelatin-doxorubicin (GD) and PEGylated gelatin-doxorubicin (PGD) nanoparticles were designed and their feasibilities as an anti-cancer drug were evaluated. The sizes of GD and PGD nanoparticles were about 135 and 250 nm, respectively, and they retained their structures for 2 days in PBS. Both GD and PGD had much lower cytotoxicity in vitro and in vivo than doxorubicin (DOX) at equivalent concentrations. However, PGD significantly inhibited tumor growth compared to the control and DOX treated group, and GD moderately suppressed tumor growth compared with the control but the suppressing effect of GD did not exceed that of DOX. And GD and PGD both remarkably suppressed pulmonary metastasis. We conclude that PGD is a potential cancer therapeutic, due to its excellent anti-tumor and anti-metastatic effects and low systemic toxicity.

Molecular-Docking-Guided Design, Synthesis, and Biologic Evaluation of Radioiodinated Quinazolinone Prodrugs

Enzyme-mediated cancer imaging and therapy (EMCIT) is a novel approach in which radioactive water-soluble molecules are precipitated in vivo following their hydrolysis by extracellular enzymes overexpressed by cancer cells. AutoDock 3.0 was used to model the interaction-binding between a series of iodinated quinazolinone derivatives and human placental alkaline phosphatase (PLAP, crystal structure in the Protein Data Bank) and to assess the effects of structural modification of the derivatives. Ammonium 2-(2',4'-diphosphoryloxyphenyl)-6-iodo-4-(3H)-quinazolinone (IQ2-P,4-P), having the most favorable calculated inhibition constant, was synthesized and characterized. Concentration-dependent, PLAP-mediated conversion of IQ2-P,4-P (4)/125IQ2-P,4-P (6) to water-insoluble 2-(2',4'-dihydroxyphenyl)-6-[127I/125I]iodo-4-(3H)-quinazolinone (127IQ2-OH,4-OH (2)/125IQ2-OH,4-OH (7)) was observed in solution. Autoradiography indicated that 6 is hydrolyzed by human cancer cells and the resulting 7 precipitates on exterior cell surfaces. Biodistribution studies in mice demonstrated that 6 is minimally retained by normal tissues. The findings support the validity of the EMCIT approach.

Tumor-activated prodrugs--a new approach to cancer therapy

Systemic cytotoxic (antiproliferative) anticancer drugs rely primarily for their therapeutic effect on cytokinetic differences between cancer and normal cells. One approach aimed at improving the selectivity of tumor cell killing by such compounds is the use of less toxic prodrug forms that can be selectively activated in tumor tissue (tumor-activated prodrugs; TAP). There are several mechanisms potentially exploitable for the selective activation of TAP. Some utilize unique aspects of tumor physiology such as selective enzyme expression or hypoxia. Others are based on tumor-specific delivery techniques, including activation of prodrugs by exogenous enzymes delivered to tumor cells via monoclonal antibodies (ADEPT) or generated in tumor cells from DNA constructs containing the corresponding gene (GDEPT). Whichever activating mechanism is used, only a small proportion of the tumor cells are likely to be competent to activate the prodrug. Therefore, TAP need to fully exploit these "activator" cells by being capable of killing activation-incompetent cells as well via a "bystander effect." A wide variety of chemistries have been explored for the selective activation of TAP. Examples are given of the most important-the reduction of quinones, N-oxides, and nitroaromatics by endogenous enzymes or radiation; the cleavage of amides by endogenous peptidases; and hydrolytic metabolism by a variety of exogenous enzymes, including phosphatases, kinases, amidases, and glycosidases.

The role of pro-drug therapy in the treatment of cancer

The administration of anti-cancer agents is currently associated with significant toxicity and lack of tumour specificity. Prodrugs are being designed to favourably alter the therapeutic index of these agents by improving their efficacy and reducing toxicity. Progress in the development of prodrugs including the cytotoxic agents most commonly used in cancer treatments namely 5-fluorouracil (5-FU), the anthracyclines, paclitaxel and platinum will be described. Many of these agents are at an early stage of development: however, this article will also describe those which have already made an impact in the clinic. It is likely that future improvements in care will come from refinement of the drugs already well established in clinical practice. In addition, this technology could be applied to novel agents with alternative cellular targets such as those involved in angiogenesis or in conferring metastatic potential. Thus, lessons learned with standard drugs may be applicable across a wider spectrum of therapeutics.

Drug-targeting strategies in cancer therapy

Genetic changes in cell-cycle, apoptotic, and survival pathways cause tumorigenesis, leading to significant phenotypic changes in transformed cells. These changes in the tumor environment - elevated expression of surface proteases, increased angiogenesis and glucuronidase activity - can be taken advantage of to improve the therapeutic index of existing cancer therapies. Targeting cytotoxics to tumor cells by enzymatic activation is a promising strategy for improving chemotherapeutics.

Superior therapeutic efficacy of N-L-leucyl-doxorubicin versus doxorubicin in human melanoma xenografts correlates with higher tumour concentrations of free drug

N-L-leucyl-doxorubicin (Leu-DOX), a prodrug of doxorubicin (DOX), has previously shown antitumour activity against human ovarian, breast and lung carcinomas in nude mice. In the present study, the efficacy of Leu-DOX was compared with free DOX in inhibiting the growth of four DOX-sensitive and -resistant malignant melanoma xenografts. In an attempt to elucidate mechanisms underlying any differential effect, a sensitive high-performance liquid chromatography (HPLC) method was established for measuring plasma and tumour concentrations of the two drugs and their main metabolites. Leu-DOX was more effective than free DOX in inhibiting xenograft growth. At equitoxic intravenous doses of Leu-DOX (28 mg/kg) and DOX (8 mg/kg) administered to tumour-bearing nude mice, comparable levels of DOX were found in plasma, whereas differences were seen in tumour tissue concentrations. Thus, in animals carrying highly sensitive (LOX) and resistant (THX) melanomas, higher tumour concentrations of free DOX were observed in the Leu-DOX treated group from 24 up to 240 h after drug injection. Notably, the difference in drug-induced tumour growth inhibition correlated with differences in tumour exposure to free DOX, assessed as area under the curve (AUC) calculated over the first 48 h. In conclusion, the results confirm the prodrug nature of Leu-DOX and provide a possible explanation for its increased antitumour efficacy.