PM3PY2 Cancer Drug Discovery Natural Products PDF

Reading School of Pharmacy PM3PY2 CANCER DRUG DISCOVERY AND DEVELOPMENT Professor Helen Osborn Email : [email protected] Copyright University of Reading LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  AIMS OF LECTURES To...

Reading School of Pharmacy PM3PY2 CANCER DRUG DISCOVERY AND DEVELOPMENT Professor Helen Osborn Email : [email protected] Copyright University of Reading LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  AIMS OF LECTURES To exemplify the importance of plants and natural products in drug discovery for cancer. To illustrate how your previous knowledge of chemistry can be used in the development of drugs from natural products, eg based on functional group chemistry, prodrugs, semi- synthesis, structure activity relationships, total synthesis. To highlight advantages and disadvantages of natural products within drug discovery. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  AIMS OF LECTURES To provide opportunities to integrate your knowledge from other Part 3 modules, and to consolidate knowledge from Part 1 and 2 modules. To demonstrate how the science of pharmacy is applied in the design and development of medicines. To identify ways to contribute to research and development activities to improve health outcomes. To develop knowledge to allow you to communicate with patients about their prescribed treatment. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  INTRODUCTION Cancer is a major cause of death within the world’s population and the number of new cases, as well as the number of individuals living with cancer, is expanding continuously. For many cancers there are simply no efficient methods for their treatment, hence there is an urgent need to develop new therapies with minimal side effects. Due to the enormous propensity of plants that synthesise mixtures of structurally diverse bioactive compounds, the plant kingdom is potentially a very diverse source of chemical constituents with tumour cytotoxic activity. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  PLANTS AS SOURCES OF ANTICANCER DRUGS Plants have been an excellent source of anticancer drugs, and much anecdotal evidence remains for their use against tumours in traditional systems of medicine. The most comprehensive study conducted on natural anticancer agents was during the 1960s-1980s, by the National Cancer Institute (NCI), a US government agency. Screening of libraries of compounds is still undertaken for free by the NCI. Between 1960 and 1982, 114,000 extracts from 35,000 species were screened against a number of tumour types. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT EXAMPLES OF NATURAL PRODUCT BASED ANTICANCER DRUGS Examples include: vinblastine (Velban), vincristine (Oncovin), etoposide, teniposide, taxol (Paclitaxel), navelbine (Vinorelbine), taxotere (Docetaxel), topotecan (Hycamtin) and irinotecan (Camptostar). LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT SPECIFIC EXAMPLES TO BE DISCUSSED  Camptothecin  Taxol  Podyphyllotoxin  Vinca alkaloids  Alkaloids  Cyanogenic alkaloids LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT What do you consider to be the main challenges of developing drugs based on natural products? What advantages does this approach offer? LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT 1. THE STORY OF CAMPTOTHECIN In 1957, 1000 ethanolic plant extracts screened at the Cancer Chemotherapy National Centre (US) – extracts of Camptotheca acuminata (Nyssaceae) had high activity. Collected 20 Kg of wood and bark for extraction - found to be active against a mouse leukaemia life prolongation assay - unusual to find activity in this model. Very slow laborious process – 3 months to get assay results back – compared with about 12 hrs today! LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT CAMPTOTHECIN EXTRACTION PROCESS LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT CAMPTOTHECIN, FROM CAMPTOTHECA ACUMINATA Note unique structure: an a-hydroxylactone (ester) alkaloid - highly unsaturated. Lactone (ester) group is important for activity but at blood pH it is in equilibrium with the less active ring opened carboxylate structure. Introducing substituents on to the rings (by synthesis) can alter the relative binding affinities of these structures to serum albumin such that the level of lactone (ester) present is altered accordingly. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  CAMPTOTHECIN: MECHANISM OF ACTION  Interest in CPT and its analogues low until in 1985 it was discovered that camptothecin works by a unique mechanism –inhibition of topoisomerase I (T-I), which is implicated in DNA transactions: replication, transcription and recombination.  CPT and analogues bind to a complex formed by DNA and T-I.  Many new compounds made.  9-Amino- and 10,11 methylenedioxy- compounds had great potency against human colon cancer xenografts in nude mice. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  NEW DERIVATIVES OF CAMPTOTHECIN  Much work done by Pharma companies on these compounds; a number of new drugs available as a result:  Irinotecan approved in 1994 (US): reduced toxicity cf that of CPT – used to treat metastatic colorectal cancer and effective against lung cancer and leukaemias.  100 times more soluble than CPT.  Topotecan (1996) – metastatic ovarian cancer – prodrug – metabolized in vivo to phenolic T-I inhibitor (1000 times more potent) – both drugs are hydrochloride salts of amines. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  2. TAXOL: ANOTHER STORY OF THE NCI PROGRAMME  1962: 650 plant samples collected from California, Washington and Oregon for NCI screening programme by Dr Hartwell – one of these was bark, twigs, needles and fruit of the Pacific Yew, Taxus brevifolia (Taxaceae family), a slow growing tree common along western coast of the USA.  Extracts tested at RTI.  Related to English yew, Taxus baccata, which is now also used. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  ISOLATION OF TAXOL  146 grams of extract subjected to partitioning between many aqueous and organic solvents, with monitoring activity at all stages (400 partitions!).  Finally 0.5 grams isolated: 0.004% from 12 kilos of material!  The name taxol assigned before structure known (evident that it contained hydroxyl groups).  Active against solid tumours and highly active against leukaemia models in a life prolongation assay, plus melanoma activity.  Structure elucidation – UV, IR, MS –not many NMR techniques available in the early 1960’s – extremely difficult task. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  TAXOL STRUCTURE * * * * * * * * * * *  Ester and amide side chains unusual.  Highly functionalised molecule: esters, oxetane, hydroxyls, amide, ketone and unsaturation.  Large number of chiral centres (asymmetry): very difficult to synthesise, achieved in 1994 - 26 steps! Not feasible for production.  For antitumour activity it is essential that ester group at C-13 is present – the tetraol (hydrolysed product) is inactive. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  TAXOL- NEARLY LOST!  RTI work on taxol ended in 1971 with publication of structure paper: Wall and co-workers tried to get the NCI to do further work…….. but  NCI replied taxol concentration in plant too low, extraction and isolation too difficult and tree supply limited……  Then two developments re-ignited interest in this compound – activity in a melanoma cell line and discovery of its unique mode of action  Susan Horowitz’s group discovered that whilst taxol inhibited mitosis, it stabilized microtubules and inhibited depolymerization back to tubulin – a new mechanism for anticancer activity LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  TAXOL REPRIEVED!  This was important in making the argument that because taxol had a unique structure - and now a unique mechanism of action - it SHOULD be a development candidate.  Much progress following these results - phase I and II conducted between 1983 and 1986 and other derivatives of taxol appeared, e.g. taxotere and docetaxel.  Supply issues overcome with semi-synthesis of taxol – conversion of a metabolite present in larger amounts in the needles – a renewable resource.  Taxol (paclitaxel) approved in 1993 marketed by BMS for ovarian cancer and secondary treatment for breast and NSC lung cancers, and now others. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  SEMI-SYNTHETIC APPROACH Four mature yew trees would have to be cut down to obtain enough taxol to treat one patient. Semi-synthetic procedures involve isolating a biosynthetic intermediate from the natural source rather than the final taxol compound itself. The intermediate can then be converted to the final product by conventional synthesis. For taxol, this involves extracting 10-deacetylbaccatin III from the needles of the yew tree, then carrying out a four stage synthesis. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT SEMI-SYNTHETIC APPROACH LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT SEMI-SYNTHETIC APPROACH In general, not all drugs can be fully synthesised, many have complex structures that are too difficult and expensive to synthesise on an industrial scale. Often the active components can be obtained from their natural source but this can be tedious, time consuming and expensive, as well as wasteful on natural resources. Researchers can look to develop them from biosynthetic intermediates using a semi-synthetic approach. Examples include penicillin, morphine and paclitaxel (Taxol). LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT SEMI-SYNTHETIC APPROACH Semi-synthesis can overcome these problems by extracting an intermediate that is made by nature (a biosynthetic intermediate) from the natural source rather than the final compound. This biosynthetic intermediate often has many of the structural features / chiral centres already in place. Chemical methods can then be used to convert this naturally occurring biosynthetic intermediate into the required final product. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT ADVANTAGES OF SEMI- SYNTHESIS The intermediate may be more easily extracted in higher yield than the final product itself. It is possible to synthesise analogues of the final product (also see penicillin lectures PM3PY3) to probe structure– activity relationships (SARs). Remember definition of SAR – a process which determines the requirements of the pharmacophore through the preparation of libraries of compounds LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  ADVANTAGES OF SEMI- SYNTHESIS It is possible to synthesise analogues of the final product and optimise pharmacokinetic profiles eg solubility, bioavailability, exposure, stability, and metabolism. EG Taxoids cannot be taken orally, and have various undesirable side effects. Semi-synthetic taxoids afford better oral bioavailability, improved pharmacological properties, and activity against drug-resistant cancers. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT TOTAL CHEMICAL SYNTHESIS 2. Total chemical synthesis - this relies on chemistry to assemble the final product from more simple chemical starting materials and can be used both to access the natural product, and to synthesise a wide range of analogues. However the complexity of the syntheses can make these processes difficult to scale-up for all but the earliest stages of the drug development process. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  3. PODOPHYLLUM  Podophyllum peltatum (Berberidaceae), May-apple, Devil's apple or American mandrake: a perennial found in woodlands in Canada and Eastern U.S.  Long, thin rhizome (underground stem from which the roots grow) is the most poisonous part. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  PODOPHYLLIN AND PODOPHYLLOTOXIN  Podophyllin is the resinous extract which acts as a cell poison against cells undergoing mitosis. OH  Formerly used as a topical application for O the treatment of cancer in 19th Century. O  Podophyllin resin is highly irritant and O O unpleasant cannot be used systemically.  Main natural product is podophyllotoxin– CH3O OCH3 isolated in 1880 and structure proposed in OCH3 1932. Podophyllotoxin LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  ISOLATON OF ACTIVE COMPOUND  Podophyllin is the resinous extract which acts as a cell poison against cells undergoing mitosis.  Improvements to bioassay and analytical methods indicated that there was a highly active constituent in the mixture.  Major component of is 1, minor component – most active – shown to be 2  Much work done to synthesise compounds with same structural features of 2 LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  SEMI-SYNTHETIC ANALOGUES OF PODOPHYLLOTOXIN  Many analogues synthesised – two most important - etoposide and teniposide – 1000-fold increase in potency  Etoposide – useful for treating small cell lung cancer, testicular cancer and lymphomas - teniposide also used in the treatment of brain tumours  Podophyllotoxin R O O O prevents microtubule HO O formation OH O  Etoposide and Etoposide, R = CH3 O O teniposide inhibit Teniposide, R = O topoisomerase II – S prevents DNA CH3O OCH3 synthesis and OH replication LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  4. THE MADAGASCAR PERIWINKLE  Catharanthus roseus (Apocynaceae), the common, rosy or Madagascar periwinkle – (formerly classified as the species Vinca rosea, Lochnera rosea and Ammocallis rosea).  Originally native to Madagascar but widely cultivated for hundreds of years and can now be found growing wild in most countries including the UK.  Long history of treating a wide assortment of diseases eg diabetes, skin infections, warts. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  STRUCTURES OF VINCA ALKALOIDS  The alkaloids responsible for the activity are structurally complex: dimeric indole alkaloids vincristine and vinblastine.  These compounds can be synthesised but are too expensive to produce in this way. OH N  They work by inhibiting mitosis and by binding to tubulin, thus preventing the cell from making the N N spindles it needs to be able HO C to move its chromosomes O H around as it divides. CH3 CH3O N OCOCH3  Vinblastine is marketed as R HO COOCH3 Velban by Eli Lilly. Vincristine R = CHO Vinblastine R = CH3 LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT THERAPEUTIC USES OF THE VINCA ALKALOIDS  Vinblastine: Hodgkin's disease, lymphomas, advanced testicular cancer, advanced breast cancer, and Kaposi's sarcoma; side effects: hair loss, nausea, lowered blood cell counts, headache, stomach pain, numbness, constipation and mouth sores - bone marrow damage usually the dose-limiting factor.  Vincristine (marketed as Oncovin): acute leukaemia, Hodgkin's disease and other lymphomas.  Vindesine (a related alkaloid, marketed as Eldisine and Fildesin): melanoma and lung cancers.  Vinorelbine (semisynthetic vinca alkaloid, marketed as Navelbine): for ovarian cancer (has a be wider range of antitumor activity than the other vinca alkaloids); also used in combination with cisplatin for non-small-cell lung cancers. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  5. ALKALOIDS  Alkaloids mimicking the structures of monosaccharides are now believed to be widespread in plants and microorganisms.  Naturally occurring sugar mimics with a nitrogen in the ring are classified into five structural classes : polyhydroxylated piperidines, pyrrolidines, indolizidines, pyrrolizidines and nortropanes. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  ALKALOIDS  These sugar mimics inhibit glycosidase enzymes that are involved in a wide range of biological processes, including the synthesis of tumour associated carbohydrate antigens.  The surface carbohydrates are believed to confer metastatic properties to the tumour cells. Inhibiting the synthesis of the tumour associated carbohydrate therefore provides a potential to treat metastatic cancers. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT 6. NATURAL CYANOHYDRINS AND CYANOGENIC GLYCOSIDES  Natural cyanohydrins feature as toxic constituents in a number of plants, e.g. laurel, bitter almonds and cassava.  In the plant the cyanohydrin can be bound through an acetal linkage to a sugar, usually glucose, to produce a cyanogenic glycoside. These are NON-TOXIC. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  MODE OF ACTION AND PRODRUG DEVELOPMENT  Upon hydrolysis the glycoside breaks down to afford the sugar and HCN - TOXIC! This can occur after ingestion of plant material, via enzymatic hydrolysis.  Prodrug : deliver the cyanogenic glycoside to tumours and allow HCN to be generated around the locality of the tumour. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT What do you consider to be the main challenges of developing drugs based on natural products? What advantages does this approach offer? LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT  CONCLUSIONS Plants provide a bank of rich, complex and highly varied structures not all of which can be synthesised in laboratories. Plants have always been a rich source of lead compounds and this is particularly true for anti-cancer therapies. Many of these lead compounds are useful drugs in themselves and others have been the basis for synthetic drugs, eg through semi-synthesis. There are likely to be many exciting new lead compounds awaiting discovery within plants for discovery as new anti-cancer agents, perhaps with novel modes of action. LIMITLESS POTENTIAL | LIMITLESS OPPORTUNITIES | LIMITLESS IMPACT