PHL 616 Drug Discovery & Development Fifth Lecture By Abdelkader Ashour, Ph.D. Phone:

 Imatinib (Gleevec), contd.  Tyrosine kinases are enzymes that transfer phosphate from ATP to tyrosine residues on substrate proteins that in turn regulate cellular processes such as proliferation, differentiation and survival  Therefore, it is not surprising that deregulated tyrosine kinase activity has a central role in malignant transformation. This has also made tyrosine kinases attractive therapeutic targets for pharmacologic inhibition Creation of a new drug, Case histories  Imatinib (signal transduction inhibitor 571; STI571, formerly CGP57148B) is a rationally designed abl-specific tyrosine kinase inhibitor  On this basis, the oncology team of Ciba-Geigy (later Novartis) began a project seeking specific inhibitors of Abl kinase

 Imatinib (Gleevec), contd.  Ciba-Geigy team began a project seeking specific inhibitors of Abl kinase by developing routine biochemical assays for Abl and other kinases  Screening of synthetic compound libraries showed that compounds of the 2-phenylaminopyrimidine class showed selectivity in blocking Abl and PDGF-receptor kinases  Systematic chemical derivatization led to the synthesis of imatinib in 1992, roughly 8 years after starting the project Creation of a new drug, Case histories  Importantly, it also inhibited the growth in culture of peripheral blood or bone marrow cells from CML patients This was valuable, as it is rarely possible to carry out such ex vivo tests on material from patients – normally, it is necessary to wait until the compound enters Phase II trials before any evidence relating to clinical efficacy emerges  Imatinib had no major pharmacokinetic or toxicological shortcomings, and was highly effective in suppressing the growth of cells engineered to express Bcr-Abl, and of human tumor cells transplanted into mice

 Imatinib (Gleevec), contd.  On that basis the project was given high priority & an accelerated clinical trials program was devised. The first trials, beginning in 1998, were performed not on normal subjects, but on 83 CML patients who had failed to respond to treatment with interferon Creation of a new drug, Case histories  Different doses were tested in patients, and the pharmacokinetic parameters, adverse effects and clinical response were measured in parallel. The results showed an unequivocal clinical effect, with 100% of patients receiving the higher doses showing a good hematological response  As a result, and because the regulatory procedures were dispatched particularly rapidly, the drug was registered in record time, in May 2001, just 3 years after being tested for the first time in humans

 Imatinib (Gleevec), contd.  Why was the imatinib project successful? 1.The selection of a precisely defined molecular target which was known to be disease relevant, and was amenable to modern assay technologies 2.Setting up the various kinase assays took 4–5 years, but thereafter screening produced the lead series of compounds rather quickly. Imatinib itself was made within 4 years of starting the screening program 3.Avoiding the pitfalls of pharmacokinetics and toxicology, which so often hinder development, was very fortunate Creation of a new drug, Case histories 4.The speed of clinical development and registration was exceptional, because CML is resistant to conventional anticancer drugs, and so imatinib did not have to be compared with other treatments 5. The designation of imatinib as an ‘orphan drug’, based on the rarity of CML, allowed the trials program to be simplified and accelerated 6.Its action is readily monitored by hematological tests, permitting a rapid clinical readout. The therapeutic effect of the drug on circulating white cells is directly related to its plasma level, which is often not the case for drugs acting on solid tumors

 Trastuzumab (Herceptin)  Trastuzumab is a humanized murine monoclonal antibody that binds specifically to the extracellular domain of the human epidermal growth factor (EGF) receptor 2 (HER2) protein  The EGF family of receptor tyrosine kinases comprises four members: 1.HER1 (EGFR/ErbB1) 2.HER2 (neu/ErbB2) 3.HER3 (ErbB3) 4.HER4 (ErbB4) Creation of a new drug, Case histories  The Her2 receptor was first cloned in 1985, and 2 years later it was found to be strongly over-expressed in the most aggressive breast cancers HER2 is a key contributor to normal cell growth and differentiation. However, when overexpressed, it is associated with neoplastic transformation of cells  Approximately 15% to 20% of breast cancers show HER2 overexpression  HER2-positive malignancies have a significantly more aggressive disease course and lead to a worse clinical outcome, including shortened overall survival, compared with those that do not overexpress HER2

Kinase-linked Receptors, Growth Factor Receptors Activation of Ras GDP/GTP Exchange Activation Binding of SH2-domain protein (Grb2) Tyrosine residue Conformation change Dimerisation Tyrosine autophosphrylation Phosphorylation of Grb2 Raf Mek MAP kinase Various transcription factors GTP NUCLEUS Gene Transcription  Agonist binding leads to dimerisation and autophosphorylation of the intracellular domain of each receptor  SH2 domain proteins, Grb2, then bind to the phosphorylated receptor and are themselves phosphorylated  Ras, which is a proto-oncogene product, functions like a G-protein, and conveys the signal (by GDP/GTP exchange)  Activation of Ras in turn activates Raf, which is the first of a sequence of three kinases, each of which phosphorylates, and activates, the next in line  The last of these, mitogen-activated protein (MAP) kinase, phosphorylates one or more transcription factors that initiate gene expression, resulting in a variety of cellular responses, including cell division Ras Grb2 MEMBRANE Sos + A

 Trastuzumab (Herceptin)  Trastuzumab is a humanized murine monoclonal antibody that binds specifically to the extracellular domain of the human epidermal growth factor (EGF) receptor 2 (HER2) protein Creation of a new drug, Case histories

Creation of a new drug, Trastuzumab

 Trastuzumab (Herceptin), contd.  This project, which took 8 years from compound discovery to registration, shows the speed with which biopharmaceuticals, under the right conditions, can be developed  Genentech used its in-house technology to develop a humanized mouse monoclonal antibody that blocked the function of the receptor and suppressed the proliferation of receptor-bearing cells Creation of a new drug, Case histories  Compared with conventional lead finding and lead optimization of synthetic molecules, this took very little time – only 2 years from the start of the project  Antibodies generally exhibit much simpler and more predictable pharmacological effects than synthetic compounds, and run into fewer problems with chemical development, formulation and toxicology, so that trastuzumab was able to enter Phase I within 2 years  Clear-cut efficacy was evident in Phase II, and the rest of the clinical development was rapid and straightforward  Given the right circumstances, biopharmaceuticals can be developed more quickly and more cheaply than conventional drugs

Creation of a new drug, Wrap up