Challenges And Rewards Of Prodrugs - Case Study Example

Challenges And Rewards Of Prodrugs Introduction: Prodrugs are basically the pharmacological substances or drugs which are administrated in less active state and which are activated in the second stage with the help of some specific enzymes to make it active to use it as one of the most advanced drug in the field of cancer treatment. When these effective prodrugs get administered they get metabolized into an active metabolite. These prodrugs are basically bio-reversible pharmacological factors of molecules which have some obstacles to its importance as effective drugs due to the inactive pretty in the first stage. The specific design and the use of the prodrugs in the medical industry can treated as a very unique problem solving the problems related to the conventional treatment of the cancer. The concept of prodrug is not a new bio-medical phenomenon, according to Albert in his book, Selective Toxicity, the concept of prodrug is been discussed in vast manner. The term prodrug was first used in 1958. The prodrugs are very much important factors in the advanced cancer treatment. During 1993-2003 the percentage of prodrug patents on the therapeutic area of cancer treatment was almost 37. [1] Produgs have proven themselves as the vital factors in cancer treatment and the medicinal values of these drugs are enormous, in a recent trend they are used in a very positive manner in pharmacology. However, they have many problems which are also very important to overcome to approve the prodrug therapy as the best possible therapy in the medical and pharmacological practice in the treatment of the most vicious diseases, and that is cancer. Why to use prodrug? Prodrug activated cancer therapy is treated as one of the successful cancer therapy in the recent study of the clinical research of the cancer treatment [2]. It is very important to understand why to use prodrugs, while there are other conventional therapies that are available towards the effected cells or tissues in the cancer treatment. Prodrug is basically primarily inactive pharmacological compound which goes under the vivo or various chemicals, mostly enzymatic conversions to an active state to produce the positive therapeutic effects. The main reason why the importance of prodrug is vital is, the use of prodrug overcomes the barrier problem such as improving the absorption, it lowers the pain during the injection, which improves the patient’s acceptability, it alters metabolism and in many ways it helps the medical science for the advanced and improve treatments. The pivotal goal of this new advanced therapy is to improve the generation of the cytotoxic drug metabolizes locally, within their existence at the tumor cells. The existing anticancer agents which are used in different treatment of cancer therapy for the monstrous desease are having many side effects. The cytotoxic drugs which are used as the anticancer agents are always dosed in short duration high dose cycle rather than giving continuous low dose. This process is design to kill many numbers of cells with high dose of drugs. During this period it allows the body to get recovered gradually from the side effects for the chemotherapy [3] the main side effects are shown in the hair follicle and gut mucosa etc. Mucosal cell can show many signs of the toxic side effects due to the effect of cancer drugs, such as chemotherapy. The toxicity as the side effect can show many painful problems as anorexia, diarrhea, vomiting tendency or nausea etc. The hair follicle cells are also affected by the side effects. Alopecia or the temporary hair fall may be sited, but most of the time this is temporary. The hair follicle cells are one of the rapidly dividing cells in the human body and the cell division is very fast for these particular cells. Now the main objective of the chemotherapy is to kill the malignant cells, due to this factor the main problem occurs while the drugs used in chemotherapy can’t differentiate between the tumor cell, that is the malignant cells and the rapidly growing hair follicle cells so unwontedly they destroy the healthy hair follicle which comes out as rapid hair loss. The hair loss depends on the drug used and their dose and definitely the duration of the treatment. It has been seen that is the hair loss is going to happen for the chemotherapy then it would be effective after 2 or 3 weeks after the treatment. [4] The most of the anticancer drugs are cytotoxic agents at the primary stage and they interact actively with some of the medical processes in our body such as cell division, DNA replication, repairing effects etc, so it is very obvious that due to this particular interference towards the normal biological processes many side effects would show up. The produgs are not very prominent in cytotoxic activity at the primary stage but they can be converted to active drugs with the help of some enzymes during the treatment of the patient. Prodrugs are activated by few important enzymes and one of the most important enzymes is the NQO1. NQO1 is one of the unusual enzymes which can utilize both NADPH and NADH as a cofactor [5]. It is clinically vital and accepted as of its ability to activate the producgs. For an example, the cytotoxic antitumor produgs. such as mitomycin. NQO1 is also very important for its protection ability for the tissues against the carcinogenic factors, cytotoxic and other effects of the semiquinones or the effective pharmacological factors produced from the metabolism of the quinines which are produced endogenously. Types of prodrigs: There are several types and classifications of prodgugs. It is expected that after the treatment the prodrugs would be taking the active form. This fact has been termed as Drug Latentiation to give the significant importance to the rational design approach. The carrier-linked prodrug is something which contains an active drug joined with a carrier group that can be removed with the help of enzymes. The effective bond to the carrier group must be strong enough to allow the active drug to release efficiently in vivo. Carrier linked prodrugs can be of different kinds, they can be subdivided into bypartate, tripartate and mutual prodrugs. [6] Bioprecursor produg is a biochemical compound which is metabolized by molecular modification. For example, if the associated drug is consist of carboxylic acid group the bioprecursor prodrug would be primary amine which is metabolized by oxidation process to the aldehyde, which is metabolized to carboxylic acid drug. This is not same as the carrier linked prodrug. The bioprecursor prodrugs have a different biochemical structure which cannot be transferred or rather converted to active drug by cleavage of a group from the prodrug. It is very important to remember some facts while designing the prodrugs. It should be kept in mind that the desing of the prodrug should be very specific, it means, a prodrug which is designed for rat metabolism might not be effective in human. Prodrug therapy: Enzyme treatments have been developed - prodrug to provide enzymes for the external Tumor tissue to convert the selective use of relatively non-toxic prodrug to an active Drugs. This leads to higher local drug concentration in the tumor, and improve Anti-tumor effect. In addition, this reduces the concentration of drug regulatory under the reduction of unwanted side effects that accompany the traditional cancer Chemotherapy. Enzyme can either be delivered by antibody enzyme Fusion protein (antibodies directed enzyme therapy - prodrug, Maher) or by Vector carrying the gene encoding for the enzyme of Foreign Affairs (genes oriented Enzyme - prodrug therapy, GDEPT). When being used as a viral vector to deliver genes, Reference is also made of the latter on the treatment of viral, according to the directives of the enzyme-prodrug therapy ADEPT (Antibody Directed Enzyme Prodrug Therapy): There are many strategies are been explored for the perfect treatment of cancer, the chemotherapy, critical surgeries and obviously treatment through radiation. However, for the effective toxicity and prominent side effects the new treatment process is been explored. The ADEPT is one of the successful processes where few very important enzymes are incorporated in the active prodrug which might ease out the effect of the side effects of the chemotherapy. The ADEPT is a two step process to activate the prodrug for the treatment of the cancer. A conjugate of enzyme with fragmented antibody is administered to the body and it is localized in the tumor site as the fragmented antibody gets recognized by the antigen on the surface of tumor cell membrane. This way the antibody and enzyme get settled in the tumor. After it localized in the tumor site and differentiated from the other tissue and blood to which the particular antibody doesn’t get bind up. The prodrug is not activated in the normal tissue or blood. Prodrugs are activated in the tumor site after they get converted to the active cytotoxic. There are many advantages of the ADEPT. The main advantage is the risk is less and the risk of second therapy which normally happens in the conventional cancer therapy is reduced. ADEPT component: The ADEPT does work by a system which consist of prodrug, antibody-enzyme conjugate (AEC) and antigen on the cancer cell. The optimum amount of each component is very important for the better outcome. The details of the components are given bellow. 1. Prodrug: The usage of prodrug is very much recommended for the cancer treatment. The active drug can be mixed in the blood and it can generate toxicity to the normal cell but the prodrug is far better than the active drug as they protect the tissues from the toxicity. However, these prodrugs are inactive in the primary stages they get activated with the help of the proper enzyme. For the proper treatment the low concentration of required active drug is required at the tumor site, and due to this fact the ADEPT allows to use very potent alkylating agent. For an example, nitrogen mustard which is too toxic to be used in the traditional cancer treatment therapy. 2. Antigen: Antigens exist as secreted antigens these secreted antigens normally have longer dwell time. In one report of breast cancer, the antigen target was membrane bound, so the AEC was there at the tumor site for a longer period. It was suggested that these secreted antigenic targets are much better target for the ADEPT. The density of the antigen expression is very much important and a very critical factor. 3. Antibody component: The immunogenicity of the antibody should be avoided, and that is why the monoclonal antibody was produced which was used for the ADEPT. Even after having the great potential about the proper targeting of monoclonal antibody sometimes some problem occurs which have been recorded and they are, slow elimination of the monoclonal antibody from blood and very poor vascular permeability, cross reactivity with the normal tissues and rapid metabolism of the monoclonal antibody conjugates. 4. Enzyme component: An enzyme should be capable of bioactivating prodrug. The ADEPT enzymes are required to be more active within the host’s cell. The main factors which help in selecting the proper enzymes are low immunogenicity, pH optimum, small size, high stability in VIVO and merely any expression on the human cells or the human tissues. 5. Antibody enzyme conjugate: The conjugation of the antibody and the enzyme has been described by the conventional linkage. The thiother shows a great stability in vivo comare to disulfide linkage. However, the critical and important information required was the relative rate of the clearance of the enzyme activity from tumor and normal tissues. Problem related to ADEPT: There are many potential problems are there in ADEPT, few of them are, 1. Poor penetration of tumor by the antibody-enzyme conjugates, because the antibodies are larger in size than the conventional anticancer drugs. The bigger molecules take much more time to puncture the tumor masses than the smaller one. 2. Non human enzymes sometimes could be immunogenic; the host prevents the repeated treatment with AEC. So in order to provide more than one treatment the AEC has to be nonimmunogenic. 3. As the conjugate is made of the human enzyme, so it can lead to lack of specificity. 5. the most problematic fact is it might take more time to clear the AEC from the blood and the normal tissues may be very slow. GDEPT (Gene Directed Enzyme Prodrug therapy): There are many therapies available for the treatment of cancer the most well know is the chemotherapy, but the chemotherapeutic drug doses show many strong side effects, to overcome this problem there are many new treatments are getting practiced. One of the well known therapies is the gene transfer therapy. Among many patients the gene suitable for the gene therapy are those which encode produgs-activation enzymes. This form of cancer therapy is called as the gene-directed enzyme prodrug therapy (GDEPT). In the conventional cancer therapy that is the chemotherapy, this is the process where the prodrug gets activated on the cells which might be far away from the tumor cell. The advantage of the GDEPT is that the activation of the pharmacologically inactive prodrug happens in tumor cells get balanced with the produg activation enzyme of suicide gene. The proper combination of prodrug ganciclovir and the harpes simplex virus thymidine Kinase (HSV-TK) enzyme presents the GDEPT Prodrugs. Like the other therapies the gene transfer therapy or the GDPET therapy is a two step process. In the first step is a directed gene encoding prodrug activation of the enzyme to tumor tissue. In the second step is to manage the prodrug and activated at a later time by prodrug. The success or failure of GDEPT Strategies depends not only on choosing the right combination of enzyme, but prodrug Also on the efficiency of gene transfection and expression of genes in the continued Tumor tissue. Therefore, the design of appropriate delivery vectors remains the greatest challenge in suicide gene therapy. In two decades, and many of the past Methods have been developed and non-viral viral versus Altenbig. Since the development of viral vectors to transfect normal host cells and efficient Have appropriate mechanisms for the delivery of molecular genetics, it is widely the use of gene therapy to suicide. In this vector, and genes required for phase are replaced by the virus replication genes encode prodrug activation of the enzyme. In this way, it is a non-replicable virus that can infect the target cells and the introduction of genes either by integrating them into Genomic DNA of the target cell or the resident plasmid. However, this is still under the study of research. Since the virus is the evolution of parasites, they raise an immune response that reduces the clinical Use. In spite of this, has been designed a viral vector several suicide genes Treatment. Attention was focused on four types: adenoviruses, retroviruses (Including lentiviruses), adeno-associated viruses and herpes simplex type 1. One of the major challenges of viral vector is a targeting vector for efficient Cells of interest: to the success of gene therapy, genes and appropriate Must be delivered to and expressed in target cells, without harming non-target Cells. One way to obtain the expression of tumor-specific prodrug activation of the enzyme is by targeting the transcriptional profiling. In this approach enhancerpromoters tumor-specific and use copies of the genes, allowing the cancer cells only. Many tumor-specific Enhancers - promoter sequence used for detection and adenoviruses target different tissues, that adenoviral vector carrying the suicide gene controlled by the tumor specific regulatory showed elements that target both effective. However, transcriptional Transfection does not prevent targeting of healthy tissue, and toxic effects related to this disintegration of the viral vectors. Another way to achieve tumor-specific expression of suicide genes are Targeting viral vectors on the surface of cancer cells directly by using native Tropism (host range) of the viral vector. However, this Tropism native oftendoes not satisfy the need for treatment. May Tropism of origin will not be able to Cancerous tissue, specifically transfect and therefore does not need to retreat to avoid Toxic side effects. Mechanisms have been put so many different Approval to target viral vectors to specific tissues, including pseudotyping, Converter systems and genetic approach PH pH-dependent: The tumor cell has lower extracellular pH level than the other normal cell because of the inefficient clearance of the metabolic acid from the hypoxic cell. This difference of the pH value is small in practical term and it is very difficult to exploit. One of the interesting approaches is used which releases the phosphoramide mustard from the sugar acetal. Hypoxia activated: Tumor hypoxia is recognized as a unique property of cells in solid tumors, as a result of an unorganized microvascular system in tumors. The hypoxia cells in tumor reduces the response of the radiation therapy, takes the responsibility for the tumor progression. For an example, selection of the p53 mutation. The hypoxia activated prodrugs are important with one electron reductases, for an example, cytochrome p450 reductase. The one electron adducts primarily formed by these component are repeatedly back oxidized by free oxygen in the normal tissue to form the effective prodrug. Hypoxia: Hypoxia is basically a condition when the body or rather some part of body doesn’t appropriate amount of oxygen. The fluctuating oxygen supply at par with the demand level of the cell might result as hypoxia. Hypoxia selective cytotoxins are made accordingly to activate preferentially to toxic species in the cellular microenvironment where the oxygen level is low in many solid tumor. Why are Nitroarmatic groups good in prodrugs? The nitro group of different kinds of nitroaromatics has been presented to undergo till six electron reduction in the cell by the flavin containing enzymes. The nitroanion redical is one of the early intermediates in the reduction process. This reactive reduction product is responsible for the cytotoxicity. The nitroanion redical, can react with the oxygen in the molecular level. It is been suggested that activated aromatic nitrogen mustards possess the required stability for such diffusion and that appropriately deactivated bioreductive prodrug forms of these can be made, since the cytotoxicity of aromatic nitrogen mustards is very dependent on the electronic properties of the substituents on the aromatic ring. An example of this approach to HSCs is the nitro-deactivated mustard 5-[N,N-bis(2-chloroethyl)amino]-2,4-dinitrobenzamide (2a; SN 23862). It has been shown that 2a undergoes efficient cellular bioreduction of the 4-nitro function to an amino group under hypoxia, thus activating the mustard which is in a resonant position by electron release. While 2a has an IC50 of 1600 íM against AA8 cells in culture (4 h exposure), the 4-amino derivative 45 has an IC50 under similar conditions of 180 íM. The hydroxylamino derivative (44) can be prepared by radiolytic or chemical reduction of 2a, but its instability precluded an exact determination of its cytotoxicity. We have also shown7 that 2a is an effective HSC in culture, being 60-fold more potent against anoxic than aerobic Chinese hamster UV4 cells. Unlike its aziridinyl analogue (1; CB 1954),11 2a is not a substrate for the two-electron reductase DT-diaphorase,12 which can activate nitroaromatics in an oxygen-insensitive manner. Quinones: The quinines normally have the primary potential to undergo one-electron reduction (by cytochrome P450 reductase) to semiquinone radicals, which regenerate the parent quinone with other reactions. The quinones work as a carrier of cytotoxic agent. The quinones are responsible for hypoxia selectivity as well as generation of reactive center. In the recent study two groups have independently proposed as approach where the quinone moiety serves as a promoiety that would help the release of the covalently bound cytotoxic agent on the reduction of the quinone. N-Oxides: N-Oxides are highly recognized as the potential prodrugs of the nitrogenous prodrugs because they are known for undergoing deoxygenation in the cell. So they also serve as hypoxia selective prodrugs. There are very few N-Oxides which have been tasted for their character to act as hypoxia selective prodrug. Nitromin, a prodrug of the alkylating agent mechlorethamine, exhibits modest hypoxia selectivity in the cell culture as for the release of the mustard. In other hand another N-Oxide chlorambucil N-Oxide, a prodrug of chlorambucil does not show any hypoxia selectivity. The main class of N-Oxides which have shown great performance is the heteroaromatic N-Oxides. One example is SR4255 this is a member of the benzotriazene di-N-Oxides, this was generated as antimalarials. In a recent study many aspects of hypoxia selective prodrugs using produgs as an example (the cytotoxity of the SR4233 as a function of oxygen concentration) is been critically discussed. SR4233 undergoes reduction process to mono and dideoxygenated products, SR4317 and SR4330 respectively with the presence of cytochrome p450. NADPH: cytochrome p450 oxidoreductase, xanthine oxidase, DT diaphorase and sldehyde dehydrogenase. There are many evidence have been obtained which tells that the species responsible for the hypoxic cytotoxicity is the SR4233 radical, the one electron reduction product of SR4233. The formation of the redical is reversible in the presence of oxygen. In the hypoxic situation this oxidizing redical is thought to abstract ribose hydrogen in DNA which leads to the single and double strand breaks of the type that are not repairable . So it is very clear that enzymatic activation is very important for the cytotoxic effects of SR4233. What is Nitroreductase? E. coli nitroreductase is a protein found in bacteria E. coli. The E. coli nitroreductase converts the produg CB1954 which is having weak alkaline properties to its 4-hydroxylamino derivatives. The inactive prodrug gets alkylated by the alkylating agent present in the cell. Such alkylating agent is scetyle coenzyme A, this leads to a strong alkylating agent able to introduce DNA cross links which are repaired poorly by the host cell.DT diaphorase is very much responsible for the primary prodrug conversion. What is NQO1? NQO1 or the NAD(P)H quinone oxidoreductase 1 is a flavor enzyme which catalyzes two electron reduction to quinonesto hydroquinones. NQO1 is a homodimer which work in specific mechanism. NQO1 is basically located on chromosome 16q2.2. The main work of the NQO1 is to encode an enzyme used in detoxification and protection against redoxcycling. For an example NQO1 detoxifies the quinones generated from the oxidation of many aromatic hydrocarbons, few examples are benzene and bezo[a]pyrene. The enzyme is actively involved in the activation of the nitroarometics and the heterocyclic amines. Most of the studies related to NQO1 genotype and the cancer risk is contradictory. A merely high of risk for colorectal cancer was observed for the people who smoke and having NQO1*2/*2 genotype. In other side the increasing risk of lung cancer have been linked with the NQO1*1/*1 genotype, the effect of this is more prone to the people who smoke. The benzene toxicity is related to the NQO1 Polymorphism. The NQO1 genotype had no effect on DNA adducts generated by the polycyclic aromatic hydrocarbons in normal breast or lung tissue.[7]. DT-Diaphorase (DTD): The DT-diaphorase is a two- electron reductase which activates the quinines in the treatment of cancer. Mainly the chemotherapeutic quinones get activated; it is also used for lowering down the toxic side effects of the chemotherapy. One of the prototypical bioreductive drugs is Mitomycin C which is largely used in clinical research. But this is actually poor substrate for DTD and its metabolism is strictly pH dependent. The DT-diaphorae (DTD) is a flavoprotein which catalyses the two electron reduction of different redox system. As it has been proposed that this particular enzyme may have some protective nature against cancer by obstructing the formation of free radical activated toxic oxygen metabolites.it is been proposed that effect of eating cruciferous vegetables on the tumorigenesis could be due in part to DTD-mediated reduces in oxidative cellula damage.[8]. How does your prodrug get activated by NQO1? The NQO1 is a member of the NAD(P)H dehydrogenase family it encodes a cytoplasmic 2 electron reductase. This FAD binding proteins formulate the homodimers and reduces quinone to hydroquinone. The NQO1 prevents the one electron reduction of quinones which results in production of radical species. There are many cytotoxic drugs which contain the quinone moiety for an example the benzoquinone group which can be activated by the NQO1 by the fragmentation into hydroquinone. The cytotoxic drugs are basically not the most preferred prodrugs for activating the NQO1 in Human tumor. Conclusion: The cancer therapy has become one of the main criteria in the recent clinical research field. There have been many kinds of therapies are practiced in the recent time. The chemotherapy is the most well known but it is painful and it has lingering side effects on the human tissues. The key criteria of a successful therapy are, to make it successful, to reduce the repeated treatment and also to nullify the side effects. The prodrugs are the new way of treating the cancer patients. One of the main reasons why prodrugs are preferred is they protect the other tissues from the effective damages due to the chemotherapy. With the help of the proper prodrug therapy the cancer can be treated in a systematic manner. The main moto of the study would be to create effective way of treating the cancer, where the patient and the cancer can be treated in a properly managed way. And another very important aspect is, providing the safety to the patient, because cancer is a very critical disease and it requires safety as well as the proper treatment in a fast manner. It is not like any other biochemical or pharmacological treatment; it is taken as the most critical treatment ever. However, with the advancement of the prodrug therapy the medical science would be able to solve the threat of the cancer. But still it is under the observation and there is a long way to go to eradicate the life threatening facts of cancer. References. 1. Stella, Valentino J. Prodrugs: Challenges and Rewards. Springer, California (2007). 2. Walther, W. & Stein, U. Gene therapy of cancer: methods and protocols. Humana Press. New Jersey. (2000) 3 .Lu, Y & Mahato, Ram I. Pharmaceutical Perspectives of Cancer Therapeutics . Sringer, California (2009). 4. Wilkes, Gail M. Ades Terri B. American Cancer Society. Consumers guide to cancer drugs. Jones & Bartlett Learning, Massachusetts (2004). 5. Costa, Lucio G & Eaton, David L. Gene-environment interactions: fundamentals of ecogenetics. John Wiley and Sons, New Jersey. (2006). 6. Silverman, Richard B. The organic chemistry of drug design and drug action. Academic Press, Missouri (2004). 7. Waller Derek G, Renwick Andrew G & Hillier, K. Medical pharmacology and therapeutics. Elsevier Health Sciences, Philadelphia (2001). 8. Current Medicinal Chemistry. Bentham Science Publishers. Illinois. (1998). Read More