Abiraterone switches castration-resistant prostate cancer dependency from adrenal androgens towards androgen receptor variants and glucocorticoid receptor signalling.

INTRODUCTION: Castration-resistant prostate cancer (CRPC) remains dependent on androgen receptor (AR) signalling, which is largely driven by conversion of adrenal androgen precursors lasting after castration. Abiraterone, an inhibitor of the steroidogenic enzyme CYP17A1, has been demonstrated to reduce adrenal androgen synthesis and prolong CRPC patient survival. To study mechanisms of resistance to castration and abiraterone, we created coculture models using human prostate and adrenal tumours. MATERIALS AND METHODS: Castration-naïve and CRPC clones of VCaP were incubated with steroid substrates or cocultured with human adrenal cells (H295R) and treated with abiraterone or the antiandrogen enzalutamide. Male mice bearing VCaP xenografts with and without concurrent H295R xenografts were castrated and treated with placebo or abiraterone. Response was assessed by tumour growth and PSA release. Plasma and tumour steroid levels were assessed by LC/MS-MS. Quantitative polymerase chain reaction determined steroidogenic enzyme, nuclear receptor and AR target gene expression. RESULTS: In vitro, adrenal androgens induced castration-naïve and CRPC cell growth, while precursors steroids for de novo synthesis did not. In a coculture system, abiraterone blocked H295R-induced growth of VCaP cells. In vivo, H295R promoted castration-resistant VCaP growth. Abiraterone only inhibited VCaP growth or PSA production in the presence of H295R. Plasma steroid levels demonstrated CYP17A1 inhibition by abiraterone, whilst CRPC tumour tissue steroid levels showed no evidence of de novo intratumoural androgen production. Castration-resistant and abiraterone-resistant VCaP tumours had increased levels of AR, AR variants and glucocorticoid receptor (GR) resulting in equal AR target gene expression levels compared to noncastrate tumours. CONCLUSIONS: In our model, ligand-dependent AR-regulated regrowth of CRPC was predominantly supported via adrenal androgen precursor production while there was no evidence for intratumoural androgen synthesis. Abiraterone-resistant tumours relied on AR overexpression, expression of ligand-independent AR variants and GR signalling.

PREDECT Protocols for Complex 2D/3D Cultures.

PREDECT, a European IMI consortium, has assumed the task to generate robust 2D and 3D culture platforms. Protocols established for 2D and 3D monoculture and stromal coculture models of increasing complexity (spheroid, stirred-tank bioreactor, Matrigel- and collagen-embedded cultures) have been established between six laboratories within academia, biotech, and pharma. These models were tested using three tumor cell lines (MCF7, LNCaP, and NCI-H1437), covering three pathologies (breast, prostate, and lung), but should be readily transferable to other model systems. Fluorescent protein tagged cell lines were used for all platforms, allowing for online measurement of growth curves and drug responses to treatments. All methods, from culture setup to phenotypic characterization and gene expression profiling are described in this chapter.The adaptable methodologies and detailed protocols described here should help to include these models more readily to the drug discovery pipeline.

Protocols and characterization data for 2D, 3D, and slice-based tumor models from the PREDECT project.

Two-dimensional (2D) culture of cancer cells in vitro does not recapitulate the three-dimensional (3D) architecture, heterogeneity and complexity of human tumors. More representative models are required that better reflect key aspects of tumor biology. These are essential studies of cancer biology and immunology as well as for target validation and drug discovery. The Innovative Medicines Initiative (IMI) consortium PREDECT (www.predect.eu) characterized in vitro models of three solid tumor types with the goal to capture elements of tumor complexity and heterogeneity. 2D culture and 3D mono- and stromal co-cultures of increasing complexity, and precision-cut tumor slice models were established. Robust protocols for the generation of these platforms are described. Tissue microarrays were prepared from all the models, permitting immunohistochemical analysis of individual cells, capturing heterogeneity. 3D cultures were also characterized using image analysis. Detailed step-by-step protocols, exemplary datasets from the 2D, 3D, and slice models, and refined analytical methods were established and are presented.

Capturing tumor complexity in vitro: Comparative analysis of 2D and 3D tumor models for drug discovery.

Two-dimensional (2D) cell cultures growing on plastic do not recapitulate the three dimensional (3D) architecture and complexity of human tumors. More representative models are required for drug discovery and validation. Here, 2D culture and 3D mono- and stromal co-culture models of increasing complexity have been established and cross-comparisons made using three standard cell carcinoma lines: MCF7, LNCaP, NCI-H1437. Fluorescence-based growth curves, 3D image analysis, immunohistochemistry and treatment responses showed that end points differed according to cell type, stromal co-culture and culture format. The adaptable methodologies described here should guide the choice of appropriate simple and complex in vitro models.

A phase I study of BI 811283, an Aurora B kinase inhibitor, in patients with advanced solid tumors.

PURPOSE: This phase I study investigated the maximum tolerated dose (MTD), safety, pharmacokinetics, pharmacodynamics, and antitumor activity of the Aurora B kinase inhibitor BI 811283 in patients with advanced solid tumors. METHODS: BI 811283 was administered via 24-h infusion on Days 1 and 15 of a 4-week cycle (schedule A) or Day 1 of a 3-week cycle (schedule B) in a modified 3 + 3 dose-escalation design. Pharmacodynamic assessments included immunohistochemistry for phosphorylated histone H3 (pHH3) on skin biopsies to determine Aurora B kinase inhibition and plasma concentrations of caspase-cleaved CK-18 (apoptosis marker). RESULTS: A total of 121 patients were treated. The MTDs of BI 811283 were 125 mg (schedule A) and 230 mg (schedule B). Dose-limiting toxicities were primarily hematological (febrile neutropenia and grade 4 neutropenia); the most common drug-related adverse effects included neutropenia, fatigue, leukopenia, nausea, alopecia, diarrhea, and decreased appetite. A trend toward a decrease in pHH3 was observed, with increasing BI 811283 doses, indicating target engagement; there was no consistent trend regarding caspase-cleaved CK-18 levels. No objective response was observed although 19 patients in each schedule achieved clinical benefit (stable disease). CONCLUSIONS: BI 811283 demonstrated a generally manageable safety profile and disease stabilization in some patients. TRIAL REGISTRATION: EudraCT No: 2007-000191-17, ClinicalTrials.gov Identifier: NCT00701324.

Ellipsoid Segmentation Model for Analyzing Light-Attenuated 3D Confocal Image Stacks of Fluorescent Multi-Cellular Spheroids.

In oncology, two-dimensional in-vitro culture models are the standard test beds for the discovery and development of cancer treatments, but in the last decades, evidence emerged that such models have low predictive value for clinical efficacy. Therefore they are increasingly complemented by more physiologically relevant 3D models, such as spheroid micro-tumor cultures. If suitable fluorescent labels are applied, confocal 3D image stacks can characterize the structure of such volumetric cultures and, for example, cell proliferation. However, several issues hamper accurate analysis. In particular, signal attenuation within the tissue of the spheroids prevents the acquisition of a complete image for spheroids over 100 micrometers in diameter. And quantitative analysis of large 3D image data sets is challenging, creating a need for methods which can be applied to large-scale experiments and account for impeding factors. We present a robust, computationally inexpensive 2.5D method for the segmentation of spheroid cultures and for counting proliferating cells within them. The spheroids are assumed to be approximately ellipsoid in shape. They are identified from information present in the Maximum Intensity Projection (MIP) and the corresponding height view, also known as Z-buffer. It alerts the user when potential bias-introducing factors cannot be compensated for and includes a compensation for signal attenuation.

Cell lines expressing recombinant transmembrane domain-activated receptor kinases as tools for drug discovery.

Many receptor tyrosine kinases (RTKs) represent bona fide drug targets in oncology. Effective compounds are available, but treatment invariably leads to resistance, often due to RTK mutations. The discovery of second-generation inhibitors requires cellular models of resistant RTKs. An approach using artificial transmembrane domains (TMDs) to activate RTKs was explored for the rapid generation of simple, ligand-independent cellular RTK assays, including resistance mutants. The RTKs epidermal growth factor receptor (EGFR), MET, and KIT were chosen in a proof-of-concept study. Their intracellular domains were inserted into a series of expression vectors encoding artificial TMDs, and they were tested for autophosphorylation activity in transient transfection assays. Active constructs could be identified for MET and EGFR, but not for KIT. Rat1 cell pools were generated expressing the MET or EGFR constructs, and their sensitivity to reference tool compounds was compared to that of MKN-45 or A431 cells. A good correlation between natural and recombinant cells led us to build a panel of clinically relevant MET mutant cell pools, based on the wild-type construct, which were then profiled via MET autophosphorylation and soft agar assays. In summary, a platform was established that allows for the rapid generation of cellular models for RTKs and their resistance mutants.

Three-dimensional models of cancer for pharmacology and cancer cell biology: capturing tumor complexity in vitro/ex vivo.

Cancers are complex and heterogeneous pathological "organs" in a dynamic interplay with their host. Models of human cancer in vitro, used in cancer biology and drug discovery, are generally highly reductionist. These cancer models do not incorporate complexity or heterogeneity. This raises the question as to whether the cancer models' biochemical circuitry (not their genome) represents, with sufficient fidelity, a tumor in situ. Around 95% of new anticancer drugs eventually fail in clinical trial, despite robust indications of activity in existing in vitro pre-clinical models. Innovative models are required that better capture tumor biology. An important feature of all tissues, and tumors, is that cells grow in three dimensions. Advances in generating and characterizing simple and complex (with added stromal components) three-dimensional in vitro models (3D models) are reviewed in this article. The application of stirred bioreactors to permit both scale-up/scale-down of these cancer models and, importantly, methods to permit controlled changes in environment (pH, nutrients, and oxygen) are also described. The challenges of generating thin tumor slices, their utility, and potential advantages and disadvantages are discussed. These in vitro/ex vivo models represent a distinct move to capture the realities of tumor biology in situ, but significant characterization work still remains to be done in order to show that their biochemical circuitry accurately reflects that of a tumor.

Efficacy of sequential treatment with sunitinib-everolimus in an orthotopic mouse model of renal cell carcinoma.

BACKGROUND/AIM: Sequential treatment with targeted agents is standard of care for patients with metastatic renal cell carcinoma (mRCC). However, clinical data directly comparing treatment outcomes with a mammalian target of rapamycin inhibitor or a vascular endothelial growth factor-targeted agent in the second-line setting are lacking. We evaluated sequential treatment in a syngeneic, orthotopic mouse model of mRCC. MATERIALS AND METHODS: BALB/c mice were orthotopically implanted with murine RCC (RENCA) cells expressing luciferase and randomized to vehicle, sunitinib, sunitinib followed by sorafenib, or sunitinib followed by everolimus. Tumor growth and metastases were assessed by in vivo (whole body) and ex vivo (primary tumor, lung, liver) luciferase activity and necropsies, performed on day 20 or 46 for vehicle and treatment groups, respectively. RESULTS: Sunitinib followed by everolimus was associated with reduced luciferase activity and primary tumor weight and volume compared with sunitinib, and sunitinib followed by sorafenib. CONCLUSION: Sequential therapy with sunitinib followed by everolimus demonstrated significant antitumor and anti-metastatic effects.

Development of a p38δ mitogen activated protein kinase ELISA assay for the quantitative determination of inhibitor activity.

The p38 mitogen activated protein kinase (MAPK) has emerged as a target for treating inflammatory diseases, like rheumatoid arthritis (RA). Expression of p38δ is induced in rheumatoid arthritis synovial fibroblasts (RASFs) by a cytokine-independent pathway substantially different from other MAPK pathways. To identify inhibitors of p38δ MAPK, we developed a direct ELISA assay based on a previously described p38α assay for monitoring the phosphorylation of ATF-2. This work presents a straightforward assay for evaluating the potency of small-molecule inhibitors. To validate the assay under optimized conditions, we used reference compounds and achieved results comparable to published data.

Zosuquidar and an albumin-binding prodrug of zosuquidar reverse multidrug resistance in breast cancer cells of doxorubicin and an albumin-binding prodrug of doxorubicin.

The P-glycoprotein (P-gp) is a 170-kDa protein that acts as an energy dependent, transmembrane efflux pump and is encoded by the MDR1 gene. It has been shown to be responsible for multidrug resistance (MDR) in a defined subpopulation of breast cancer patients and thus represents a molecular target for circumventing MDR in this tumor indication. MDR modulators have been developed and demonstrated high selectivity for P-gp with inhibitory activities in the low nanomolar range. Although some objective responses were achieved in clinical trials, combination therapy with these MDR modulators, such as Ca2+ antagonists caused unacceptable toxicity. Targeting P-gp inhibitors to the tumor site is a mean to increase their therapeutic index, and in this context binding of tailor-made prodrugs to circulating albumin is an established technology to reduce the toxicity and enhance the efficacy of anticancer drugs. In this study, we consequently developed an acid-sensitive albumin-binding prodrug of the P-gp inhibitor zosuquidar (LY335979) in a two-step synthesis using a maleimide hydrazone linker system established in our laboratory that first introduces acetylbenzoic acid at the HO-group of zosuquidar followed by derivatization with 6-maleimidocaproyl hydrazide to form the acid-sensitive hydrazone bond. The maleimide group enables the prodrug to bind rapidly and selectively to the cysteine-34 position of endogenous albumin after intravenous administration. HPLC analysis demonstrated rapid albumin binding of the zosuquidar prodrug as well as the quantitative release of the acetylbenzoic ester derivative of zosuquidar at pH 5.0. Subsequently, its ability to circumvent MDR was tested in two doxorubicin-resistant breast carcinoma cell lines (MCF-7/ADR and MT-3/ADR). The MDR status of these cell lines can be reversed by zosuquidar which was confirmed in a rhodamine 123 assay using fluorescence microscopy and FACS analysis. Furthermore, zosuquidar as well its acid-sensitive albumin conjugate re-sensitized cells to doxorubicin as well as to an albumin-binding prodrug of doxorubicin, i.e., the 6-maleimidocaproyl hydrazone derivative of doxorubicin, achieving IC50 values in the same order of magnitude as the parental cell lines. Thus, a novel formulation of zosuquidar has been developed that could have the potential to improve the toxicity issues and tumor targeting properties of the original compound.

Generation of a conditionally transformed murine embryonic fibroblast cell line using doxycycline-dependent IGF-1R overexpression.

The insulin-like growth factor I receptor (IGF1-R) system has long been implicated in cancer and is a promising target for tumor therapy. Besides in vitro screening assays, the discovery of specific inhibitors against IGF-1R requires relevant cellular models, ideally applicable to both in vitro and in vivo studies. With this aim in mind, the authors generated an inducible cell line using the tetracycline-responsive gene expression system to mimic the effects of therapeutic inhibition of the IGF-1R both in vitro and on established tumors in vivo. Inducible overexpression of IGF-1R in murine embryonic fibroblasts was achieved and resulted in the transformation of the cells as verified by their ability to grow in soft agar and in nude mice. Continuous repression of exogenous IGF-1R expression completely prevented outgrowth of the tumors. Furthermore, induced repression of IGF-1R expression in established tumors resulted in regression of the tumors. Interestingly, however, IGF-1R-independent relapse of tumor growth was observed upon prolonged IGF-1R repression. The IGF-1R cell line generated using this approach was successfully employed to test reference small-molecule inhibitors in vitro and an IGF-1R-specific inhibitory antibody, EM164, in vivo. Besides efficacy as a read-out, phospho-AKT could be identified as a pharmacodynamic biomarker, establishing this cell line as a valuable tool for the preclinical development of IGF-1R inhibitors.

A direct ELISA assay for quantitative determination of the inhibitory potency of small molecules inhibitors for JNK3.

The c-jun N-terminal kinase 3 (JNK3) is a promising drug target for the treatment of neurological disorders. Here we report a direct ELISA including the optimization of a nonradioactive immunosorbent JNK3 activity assay to determine inhibitory potency of small-molecule inhibitors. Based on our previous JNK3 assay and our recently optimized p38α mitogen activated protein kinase (MAPK) protocol for monitoring the phosphorylation of activating-transcription factor 2 (ATF-2), we present a rapid and straightforward alternative to conventional radioactive and indirect ELISA kinase assays. To validate the assay with the optimized assay conditions we used reference compounds and achieved well comparable IC(50) results to published data. The use of a linked monoclonal antibody increased the specificity and the sensitivity of the assay, reducing the required antibody concentration by approximately 100-fold. The novel protocol is an accurate, easy-to-handle and robust screening assay for JNK3 and the assay performance was reduced from 7.5 to 3h.

Development of efficient acid cleavable multifunctional prodrugs derived from dendritic polyglycerol with a poly(ethylene glycol) shell.

In an attempt to explore the potential of dendritic systems for the development of effective anticancer drug delivery systems, we explored a simple modular approach of preparing polyglycerol doxorubicin prodrugs, with flexibility for drug loading using an acid-sensitive hydrazone linker and further post-modification with poly(ethylene glycol) shell. The resulting drug polymer conjugates showed optimal properties for in vitro and in vivo applications because of their high water solubility, an appropriate size for passive tumor targeting, a high stability at physiological conditions, pronounced acid-sensitive properties, cellular internalization, and a favorable toxicity profile. Doxorubicin polyglycerol conjugates with a high drug loading ratio showed clearly improved antitumor efficacy over doxorubicin in an ovarian xenograft tumor model (A2780) inducing transient complete remissions thus demonstrating the potential of developing efficient multifunctional dendritic drug delivery using our modular approach.

Lysophosphatidylcholine pretreatment reduces VLA-4 and P-Selectin-mediated b16.f10 melanoma cell adhesion in vitro and inhibits metastasis-like lung invasion in vivo.

Lysophosphatidylcholine (LysoPC) is an important intermediate in degradation and biosynthesis of phosphatidylcholine (PC). Reduced plasma LysoPC levels observed in patients with advanced cancer indicate a deregulation of LysoPC metabolism in metastasis. Recent data showed strong antimetastatic effects of liposomes consisting of saturated PC in a murine pancreatic metastasis model. LysoPC, generated from saturated PC after accumulation of the liposomes in tumor tissue, might be contributing to these effects. Examining effects of high local concentrations of saturated LysoPC and investigating potential molecular mechanisms, fast removal of saturated LysoPC from medium by murine B16.F10 melanoma cells and radical shifts in tumor cell membrane fatty acid (FA) composition toward saturated FAs were observed in vitro. Scanning electron microscopy revealed remarkable morphologic surface changes of LysoPC-treated tumor cells, probably causing their impaired migratory ability on fibronectin. A LysoPC concentration exceeding a threshold of about 400 µmol/L, slightly above physiologic levels, strongly reduced VLA-4-mediated binding of B16.F10 cells to VCAM-1 as well as P-selectin-dependent interaction with activated platelets, although expression levels were not altered. These findings were reflected in a syngenic intravenous lung invasion model using repeatedly ex vivo LysoPC-treated (450 µmol/L) B16.F10 cells, resulting in significantly reduced lung metastasis-like lesions (-48.3%, P = 0.006). Prior application of 50 IU unfractionated heparin further reduced lung invasion (-81.6%, P = 0.043). Our work shows for the first time that saturated LysoPC in high concentrations reduces melanoma cell adhesion in vitro and hematogeneous dissemination in vivo by direct ex vivo tumor cell targeting.

Non-invasive in vivo imaging of tumor-associated CD133/prominin.

BACKGROUND: Cancer stem cells are thought to play a pivotal role in tumor maintenance, metastasis, tumor therapy resistance and relapse. Hence, the development of methods for non-invasive in vivo detection of cancer stem cells is of great importance. METHODOLOGY/PRINCIPAL FINDINGS: Here, we describe successful in vivo detection of CD133/prominin, a cancer stem cell surface marker for a variety of tumor entities. The CD133-specific monoclonal antibody AC133.1 was used for quantitative fluorescence-based optical imaging of mouse xenograft models based on isogenic pairs of CD133 positive and negative cell lines. A first set consisted of wild-type U251 glioblastoma cells, which do not express CD133, and lentivirally transduced CD133-overexpressing U251 cells. A second set made use of HCT116 colon carcinoma cells, which uniformly express CD133 at levels comparable to primary glioblastoma stem cells, and a CD133-negative HCT116 derivative. Not surprisingly, visualization and quantification of CD133 in overexpressing U251 xenografts was successful; more importantly, however, significant differences were also found in matched HCT116 xenograft pairs, despite the lower CD133 expression levels. The binding of i.v.-injected AC133.1 antibodies to CD133 positive, but not negative, tumor cells isolated from xenografts was confirmed by flow cytometry. CONCLUSIONS/SIGNIFICANCE: Taken together, our results show that non-invasive antibody-based in vivo imaging of tumor-associated CD133 is feasible and that CD133 antibody-based tumor targeting is efficient. This should facilitate developing clinically applicable cancer stem cell imaging methods and CD133 antibody-based therapeutics.

CXCL12 mediates immunosuppression in the lymphoma microenvironment after allogeneic transplantation of hematopoietic cells.

Clinical studies indicate a role of allogeneic hematopoietic cell transplantation (alloHCT) for patients with refractory or recurrent B-cell lymphoma (BCL) indicative of a graft-versus-tumor effect. However, the relevance of local immunosuppression in the BCL microenvironment by donor-derived regulatory T cells (Treg) after alloHCT is unclear. Therefore, we studied Treg recruitment after alloHCT in different murine BCL models and the impact of lymphoma-derived chemoattractive signals. Luciferase transgenic Tregs accumulated in murine BCL microenvironment and microarray-based analysis of BCL tissues revealed increased expression of CXCL9, CXCL10, and CXCL12. In vivo blocking identified the CXCR4/CXCL12 axis as being critical for Treg attraction toward BCL. In contrast to Tregs, effector T cells displayed low levels of CXCR4 and were not affected by the pharmacologic blockade. Most important, blocking CXCR4 not only reduced Treg migration toward tumor tissue but also enhanced antitumor responses after alloHCT. CXCL12 production was dependent on antigen-presenting cells (APC) located in the lymphoma microenvironment, and their diphtheria-toxin receptor (DTR)-based depletion in CD11c.DTR-Tg mice significantly reduced Treg accumulation within BCL tissue. CXCL12 was also detected in human diffuse, large BCL tissues indicative of its potential clinical relevance. In conclusion, we demonstrate that Tregs are recruited toward BCL after alloHCT by infiltrating host APCs in a CXCL12-dependent fashion. Blocking CXCR4 enhanced antitumor effects and prolonged survival of tumor-bearing mice by reducing local Treg accumulation, indicating that CXCR4 is a potential target to interfere with tumor escape after alloHCT.

EphB4 promotes site-specific metastatic tumor cell dissemination by interacting with endothelial cell-expressed ephrinB2.

The tyrosine kinase receptor EphB4 interacts with its ephrinB2 ligand to act as a bidirectional signaling system that mediates adhesion, migration, and guidance by controlling attractive and repulsive activities. Recent findings have shown that hematopoietic cells expressing EphB4 exert adhesive functions towards endothelial cells expressing ephrinB2. We therefore hypothesized that EphB4/ephrinB2 interactions may be involved in the preferential adhesion of EphB4-expressing tumor cells to ephrinB2-expressing endothelial cells. Screening of a panel of human tumor cell lines identified EphB4 expression in nearly all analyzed tumor cell lines. Human A375 melanoma cells engineered to express either full-length EphB4 or truncated EphB4 variants which lack the cytoplasmic catalytic domain (ΔC-EphB4) adhered preferentially to ephrinB2-expressing endothelial cells. Force spectroscopy by atomic force microscopy confirmed, on the single cell level, the rapid and direct adhesive interaction between EphB4 and ephrinB2. Tumor cell trafficking experiments in vivo using sensitive luciferase detection techniques revealed significantly more EphB4-expressing A375 cells but not ΔC-EphB4-expressing or mock-transduced control cells in the lungs, the liver, and the kidneys. Correspondingly, ephrinB2 expression was detected in the microvessels of these organs. The specificity of the EphB4-mediated tumor homing phenotype was validated by blocking the EphB4/ephrinB2 interaction with soluble EphB4-Fc. Taken together, these experiments identify adhesive EphB4/ephrinB2 interactions between tumor cells and endothelial cells as a mechanism for the site-specific metastatic dissemination of tumor cells. AACR.

Development of a novel prodrug of paclitaxel that is cleaved by prostate-specific antigen: an in vitro and in vivo evaluation study.

In developed countries, prostate cancer is the third most common cause of death from cancer in men. Unfortunately, whilst accumulating clinical data have suggested that taxanes may prolong the survival in a subset of men with prostate carcinoma, the dose and duration of administration of these drugs are limited by their significant systemic toxicities due to a lack of tumour selectivity. In an attempt to improve both the water solubility and tumour-targeting properties of paclitaxel (Taxol®), we set out to develop a water soluble paclitaxel prodrug that is activated specifically by prostate-specific antigen (PSA) which is almost exclusively expressed in prostate tissue and prostate carcinoma making it an ideal molecular target for prodrug strategies. Using albumin as a drug carrier, we describe a novel albumin-binding prodrug of paclitaxel, EMC-Arg-Ser-Ser-Tyr-Tyr-Ser-Leu-PABC-paclitaxel [EMC: ε-maleimidocaproyl; PABC: p-aminobenzyloxycarbonyl] that was synthesised by reacting EMC-Arg-Ser-Ser-Tyr-Tyr-Ser-OH with H-Leu-PABC-paclitaxel. This prodrug was water soluble and was bound to endogenous and exogenous albumin. Moreover, incubation studies with PSA demonstrated that the albumin-bound form of the prodrug was cleaved rapidly at the P1-P1' scissile bond releasing the paclitaxel-dipeptide H-Ser-Leu-PABC-paclitaxel. Last but not least, due to the incorporation of a PABC self-eliminating linker, this dipeptide was rapidly degraded to liberate paclitaxel as a final cleavage product within a few hours in prostate tumour tissue homogenates. Of note was that the albumin-bound form of the prodrug was approximately three-fold more active in killing PSA-positive LNCaP cells than paclitaxel. In addition, orientating toxicity studies in mice showed that the maximum tolerated dose of the novel paclitaxel prodrug was twice that of conventional paclitaxel. When tested in vivo in an orthotopic mouse model of human prostate cancer using luciferase-transduced LNCaP LLN cells, both paclitaxel and the new paclitaxel prodrug showed distinct antitumour efficacy on the primary tumour and metastases that was significantly better than the effect of doxorubicin which was used as a comparison and showed no antitumour efficacy. The new paclitaxel prodrug (3 × 24 mg paclitaxel equivalents) showed comparable antitumour activity on the primary tumour to paclitaxel at its maximum-tolerated dose (3 × 12mg/kg), reduced circulating PSA levels and demonstrated a better antitumour effect on lung metastases but not on bone metastases.