Current Pharmaceutical Design - Volume 8, Issue 25, 2002

Improvements in our understanding of the intrinsic aberrancies in cancer cells have enabled the design and development of novel therapeutics that specifically target these changes. Among the many complex cellular pathways and mechanisms which have been unveiled by new molecular techniques, RAS-mediated signal transduction is one met with tremendous research interests. Activation of RAS initiates several signaling cascades, of which the RAS-RAF-MEK-ERK pathway is among the better delineated, and is the main focus of this review. Other cellular consequences of RAS activation including interactions with the RHO-family proteins, the PI3-kinase pathway, and other mitogen activated protein kinase cascades, will be discussed. The intricate balance and coordination of multiple RAS-mediated signals lead to ultimate effects on cell growth, differentiation, cycling and survival. Pharmacological strategies such as analog development, synthesis of small molecule inhibitors, antisense technology, and vaccine therapy have been utilized to intervene with key RAS-signaling proteins, in an attempt to provide rational therapeutic solutions in malignant diseases.

Adult acute myeloid leukemia (AML) and the myelodysplastic syndromes (MDS) remain a formidable therapeutic challenge. Although 30-40% of young adults with AML may have prolonged remissions, following either intensive chemotherapy or bone marrow transplantation, the remainder relapse and ultimately die of their disease. Older adults, or those with leukemia following an antecedent hematologic disorder such as MDS, experience only brief remissions with modern chemotherapy, and in this population the rate of long-term disease-free survival is less than 10%. Treatment of MDS consists mainly of supportive care with antibiotics and transfusion therapy, and only a small minority of patients are cured with allogeneic stem cell transplantation. As the biology of the acute leukemias and MDS has unfolded, new potential targets for arresting malignant cell growth and restoring normal hematopoiesis have been identified. This article will discuss the relevance of the ras-raf signaling pathway in the pathogenesis of myeloid leukemia, and describes some early results in the use of novel small molecule inhibitors of farnysltransferase and raf protein kinase in leukemia therapy.

Various signalling pathways can confer the malignant phenotype to a cell. Ras signalling proteins have been found to play an important role in controlling cellular growth. Raf-1 is a protein kinase that exerts its effects downstream of Ras in the mitogen-activated protein kinase pathway and is thus likely to be crucial in the development of the malignant phenotype. BAY 43-9006 is an orally administered selective inhibitor of Raf-1 and the first compound of its class to enter clinical trials. This article describes the early clinical data of BAY 43-9006 in patients with advanced, refractory solid tumours. To date, over 60 patients have been treated as part of four Phase I clinical trials. Dose levels have ranged from 50mg once weekly to 200mg twice-daily in continuous administration. The drug has been generally well tolerated with no dose limiting toxicity yet encountered. The more common toxicities have involved the gastrointestinal tract (diarrhea, nausea, abdominal cramping) and the skin (pruritus, rash, cheilitis). Pharmacokinetic evaluations have found BAY 43-9006 to have considerable interpatient variability. However, there seems to be an increase in Cmax and AUC values with increasing dose. There is no clear effect of food on bioavailability. Splitting the dose to twice-daily administration has shown increases in Cmax and AUC values but is also accompanied by considerable interpatient variability.

The drug design and discovery efforts described in the previous section led to the development of a novel, small molecule Raf-1 kinase inhibitor, BAY 43-9006, which belongs to a class that can be broadly described as bis-aryl ureas (Figure 1) [1]. BAY 43-9006 was identified during a large medicinal chemistry optimization program, and this compound was selected for further pharmacological characterization based on its potent inhibition of Raf-1 (IC50 12 nM) and its favorable kinase selectivity profile [2, 3]. In vitro and in vivo experiments were designed to demonstrate effective blockade of the Raf / MEK / ERK signaling pathway in tumor cells and for anti-tumor efficacy in human xenograft models.

Ras genes encode proteins that activate in an intracellular signaling network controlling differentiation, proliferation and cell survival. Mutated Ras oncogenes encoding proteins that are constitutively active can induce malignancies in a variety of laboratory models. In human malignancies, Ras mutations are common, having been identified in approximately 30% of cancers. Given the importance of Ras and downstream targets Raf and MEK in the development of malignancies and their frequent expression in human cancers, it is not surprising that a variety of agents disrupting signaling through Ras and downstream proteins are under development. These agents can be broadly classified structurally as small molecules and anti-sense oligonucleotides. They can be characterized functionally as those inhibiting Ras protein expression such as the oligodeoxynucleotide ISIS 2503, those inhibiting Ras processing, in particular the farnesyl transferase inhibitors R115777, SCH 66336 and BMS 214662, and those inhibiting downstream effectors Raf, such as ISIS 5132 and MEK, which is inhibited by CI-1040. The purpose of this review is to highlight recent advances in the development of these agents.

Raf kinase, an enzyme which acts downstream in the Ras signaling pathway, is involved in cancerous cell proliferation. Thus, small molecule inhibitors of Raf kinase activity may be important agents for the treatment of cancer. A novel class of Raf-1 inhibitors was discovered, using a combination of medicinal and combinatorial chemistry approaches. This effort culminated in the identification of the clinical candidate BAY 43-9006, currently undergoing Phase I clinical trials. The present review summarizes the medicinal chemistry development of ureas as highly potent inhibitors of Raf-1 kinase.

Investigators involved in the development of cancer therapeutics are testing new trial designs and endpoints in order to accommodate the perceived challenges in defining appropriate doses and schedules for further testing. Many new agents with specific molecular targets have entered clinical development or are being considered for development. While some of the agents have both toxicity and antitumour efficacy apparent at clinically achievable doses, thus the use of traditional algorithms is appropriate, others have significant clinical activity at doses considerably lower than the maximum tolerated dose. New initiatives in clinical trial design, both phase I and phase II may allow the development of appropriate plans for the development of these new molecularly targeted agents. Measures of target effect (tissue or imaging) are now commonly included in early trials of new targeted compounds, in an attempt to demonstrate proof of principle as well as guide dose selection. Phase II trial designs including novel correlative, imaging and clinical endpoints are being tested. Alternate endpoints such as progression or time to progression are being increasingly considered, and novel designs such as randomized discontinuation designs, multinomial designs and growth modulation indices are being prospectively tested. Progress in this area of early trial design are reviewed.