“Knowing is not enough; we must apply. Willing is not enough; we must do it.”—
Johann Wolfgang von Goethe (1749–1832)
In 2004, the Food and Drug Administration (FDA) released an important report entitled “Innovation/Stagnation: Challenge and Opportunity on the Critical Path to New Medical Products” (http://www.fda.gov/oc/initiatives/criticalpath/whitepaper.pdf). This “Critical Path Initiative” is the FDA’s effort to modernize the sciences through which FDA-regulated products are developed, evaluated, and manufactured. In particular, the report diagnosed the scientific reasons for the recent decrease in the number of approved innovative medical products (23 new drug approvals by the FDA in 2006) and called for a concerted effort to modernize the scientific tools (e.g., in vitro tests, computer models, qualified biomarkers, and innovative study designs). This productivity problem in the pharmaceutical industry increased during the last decade with a double pharmaceutical research and development investment associated with a worldwide decrease of the rate of submission of new chemical entities.
Paradoxically in the same time, the diagnostic industry has been the target of a number of cost-cutting exercises by governments and payers aimed at restricting the utilization of diagnostic tests. More recently, the pharmaceutical industry received similar pressures with increasing demands for proof of efficacy and safety, and cost-effectiveness. However, the pharmaceutical and diagnostic companies have found a common answer to these regulatory and financial constraints in the emerging field of theragnostics, in which the developing technologies and capabilities of the diagnostic division are applied to improve efficiency and economics of the development and marketing of new drugs. In this way, theragnostics is clearly a concept that may respond to FDA expectation.
The term theragnostics (also spelled theranostics) was probably first used by the Chief Executive Officer of PharmaNetics, John Funkhouser, in describing his company’s business model in developing diagnostic tests directly linked to the application of specific therapies. Considering diagnostics as the ability to define a disease state, Funkhouser defined theragnostics as “the ability to affect therapy or treatment of a disease state.” Hence, theragnostics is a treatment strategy for individual patients, which associates both a diagnostic test that identifies patients most likely to be helped or harmed by a new medication, and targeted drug therapy based on the test results. Clinicians and patients will embrace the theragnostics tests when they provide information that fills an essential knowledge gap deemed clinically important to the diagnosis, prognosis, treatment, and monitoring of patients with serious disease (1).
Theragnostics covers a wide range of topics that includes predictive medicine, personalized medicine, integrated medicine, and pharmacodiagnostics. Theragnostics tests differ from traditional ones (troponin, lactate, procalcitonin, blood glucose, etc) because they are based on sophisticated recent technologies. Hence, theragnostics may be considered as the possible end result of new advances made in pharmacogenomics and drug discovery using genetics, molecular biology, and microarray chips technology. As illustrated, in the FDA’s “Table of Valid Genomic Biomarkers in the Context of Approved Drug Labels,” which portrays a view on valid genomic biomarkers in the context of FDA-approved drug labels (http://www.fda.gov/cder/genomics/genomic_biomarkers_table.htm), theragnostics has three principal key applications (2):
- Identification of subgroups of patients presenting a profile likely to give a positive response to a given treatment: Targeted therapies (Efficacy).
- Identification of subgroups of patients at risk of aggravated side effects during treatment: Pharmacogenomics (Safety).
- Monitoring the response to a treatment (Efficacy and Safety).
This review describes the principles of theragnostics and its current and future applications.
Higher Efficacy With Targeted Therapies With Theragnostics
It is generally accepted as very important to have indicators that can help physicians predict which of their patients may or may not respond to therapy. However, this evidence is not always easy to generate, which in turn can lead to economic and societal concerns over the development and commercialization of very expensive drugs that can only reasonably be prescribed to a limited number of well-defined patients. The first key application of theragnostics (and probably the most important for drug industry and payers) consists in identifying the population of patients in which a therapy will be effective through the detection by theragnostics tests. Hence, for patients, theragnostics means more effective care and the possibility of avoiding useless treatments that might have harmful side effects.
In an ideal world, the new drug and the diagnostic test will be codeveloped and approved by FDA at the same time. This situation happened on September 1998, which is a key date for theragnostics, and may be considered as the date of birth of this new concept. On that date, the FDA granted simultaneous approval for both trastuzumab (Herceptin, Roche, Basil, Switzerland) for the treatment of stage IV breast cancer and the human epidermal growth factor receptor 2 (HER2) protein test (HercepTest, Dako, Carpinteria, CA) for diagnosis of HER2 overexpression. Since then, many of the significant advances in cancer and infectious disease management have focused on the development and introduction of molecularly targeted therapy based on theragnostic tests.
Tumor Protein Detection Test: The Birth of Theragnostics
Despite advances in the diagnosis and treatment of breast cancer, which is staged between I and IV, >44,000 women in the United States die each year of metastatic disease (3). Of the stage IV, approximately 25%–30% of breast cancers show significant overexpression of a growth factor receptor, HER2. Women with breast cancers that overexpress HER2 have an aggressive form of the disease with a significantly poorer overall prognosis. Amplification of HER2 has been shown to have a direct role in the pathogenesis of these cancers, thereby providing an opportunity to target a therapeutic agent directly against this alteration (4). Trastuzumab is a humanized monoclonal antibody (mAb) to HER2 which is effective by binding to the HER2 expressed on the cell surface of the tumor cells and provides a new and highly effective tool in targeting stage IV breast cancer overexpressing HER2 (4). Two diagnostic tests, HercepTest and Path Vision (Abbott, Abbott Park, IL), are used to detect susceptible tumors, which allows treatment to be limited to patients most likely to benefit from the drug. Clearly, uptake of trastuzumab into clinical practice for the treatment of HER2 overexpressing stage IV breast cancer was rapid because knowing a tumor’s status led to an actionable decision on the selection of the most appropriate treatment (3–5).
Other examples of drugs that have been coapproved along with an eligibility diagnostic test for the selection of patients are cetuximab and the eligibility test (DakoCytomation EGFR pharmDx test kit) for immunohistochemical evidence of positive epidermal growth factor receptor (EGFR) expression in colorectal carcinoma, and imatinib and all members of the tyrosine kinase inhibitor family along with the eligibility test for the expression of the t(9;22) translocation fusion gene Bcr–Abl (chronic myelogenous leukemia [CML] and Philadelphia chromosome-positive acute lymphoblastic leukemia (Phi + ALL) or the tyrosine kinase receptor c-Kit (gastrointestinal stromal tumors). Hence, in patients with Philadelphia chromosome-positive CML unresponsive to interferon, imatinib provides a significant survival advantage (6).
In contrast, erlotinib and gefitinib, which are EGFR inhibitors indicated for patients diagnosed with non–small-cell lung cancer (NSCLC), cannot be strictly considered as molecularly targeted therapies (7). Their approval by the FDA did not include a validated eligibility diagnostic test for EGFR status designed to select patients whose tumors were more likely to respond to their action. Immunohistochemistry, fluorescence in situ hybridization, and mutational analyses of the EGFR gene have all been proposed as candidates to help predict response or survival benefit from EGFR-targeted therapy in patients with NSCLC (8). However, further prospective validation from ongoing randomized studies will be needed to fully determine which assays are best to help predict patient outcome. In addition, it will be critical for these assays to undergo standardization before widespread clinical use.
Nucleic Acid-Based Tests for Targeted Therapies
For the above-mentioned molecular therapies, the target is represented by one or more tumor-specific molecular features. Among them, Philadelphia chromosome highlights the importance of theragnostics DNA-based biomarkers. The idea that genetics plays a role in therapy response has become more prevalent in recent years in a number of diseases. Identifying the presence of genetic variants or a certain set of gene expression profiles may predict response to therapy.
Human Genetic Variations for Targeted Treatment
It has only been a little >50 years since Watson and Crick discovered the structure of DNA and nearly 7 years since the human genome sequence and its first single nucleotide polymorphism map were published in the same issue of Nature (9). On the one hand, the Human Genome Project showed a high degree of similarity between the DNA sequences of two persons, defining us as a species; on the other hand, it revealed differences in DNA sequences that make each human unique. These differences are either qualitative in the form of single nucleotide polymorphisms and haplotypes or quantitative such as deletions, insertions, duplications, and large scale rearrangements like the Philadelphia chromosome (10). Genomics-based knowledge promises the ability to approach each patient as an unique biological individual, thereby completely changing our paradigms and improving efficacy. In oncology, some emerging tests support this promise.
As mentioned above, Philadelphia chromosome or Philadelphia translocation is a specific chromosomal abnormality that is associated with CML and can be easily detected (11). It is due to a reciprocal translocation designated as t(9; 22) (q34; q11), which means an exchange of genetic material between region q34 of chromosome 9 and region q11 of chromosome 22. The presence of this translocation is a highly sensitive test for CML, which was the first malignancy to be linked to a clear genetic abnormality (11). As a result of the translocation, part of the Bcr gene from chromosome 22 is fused with the ABL gene on chromosome 9. This abnormal Bcr–Abl fusion gene generates a tyrosine kinase, which is continuously active and activates a cascade of proteins that control the cell cycle, speeding up cell division and inhibits DNA repair, causing genomic instability and making the cell more susceptible to developing further genetic abnormalities. The action of the bcr–abl protein is the pathophysiologic cause of CML, leading to the development of targeted therapies (such as imatinib) that specifically inhibit the activity of the bcr–abl protein (12). These tyrosine kinase inhibitors can induce complete remissions in CML, confirming the central importance of bcr–abl as the cause of CML. Hence, advances in understanding molecular and genetic mechanisms underlying cancer ontogenesis may lead to an individualized management of the disease.
Similarly, women carrying a BRCA1 or BRCA2 germline mutation are at very high risk of breast and/or ovarian cancer. The detection of these mutations is easy and reliable using the BRCA analysis test, which has been one of the first patented DNA-based theragnostics tests. This diagnosis helps to decide for prophylactic surgery, essentially prophylactic bilateral salpingo-oophorectomy which is superior to bilateral prophylactic mastectomy and/or chemoprevention and/or intensified surveillance, including breast magnetic resonance imaging screening (13, 14).
Gene Expression Profiles for Targeted Treatment
Beyond panels of individual genetic alleles, the entire gene expression profile derived from DNA microarray studies (entire transcriptome) has the potential for adding enormous information to the analysis of biological phenotypes. With this technology, physicians can determine which genes are “turned on” and which have been “turned off” and can identify sensitive changes in the gene expression over time. Transcriptome can distinguish normal and diseased tissues, classify the various stages of the disease (severity, progression, resolution), and select patterns of gene expression with prognostic and predictive value. Similar global approaches are being developed using patterns of disease-specific single nucleotide polymorphisms, microRNA, and epigenetic changes associated with DNA methylation (15, 16).
Perhaps the most successful application of this progress has been the characterization of human cancers, including the ability to predict clinical outcomes. As a consequence, the gene expression signatures are used for the development of new targeted therapeutics based on better detection and classification of tumors. For example, differences in gene expression profile can be used to detect tumors that already demonstrate gene expression changes associated with increased risk of metastasis, to propose more aggressive treatment. A number of studies have identified prognostic and predictive gene “signatures,” whose prediction of disease outcome and response to treatment is superior to conventional prognostic indicators. Furthermore, a relatively small number of genes seems able to predict response to breast cancer or B-lymphoma therapy. Although the results are promising, further optimization and standardization of the technique and properly designed clinical trials are required before microarrays can reliably be used as tools for clinical decision making (17).
MicroRNAs and MicroRNAs Expression Profiles for Targeted Treatment
MicroRNAs (miRNAs) are small (∼22 nucleotides) noncoding RNAs that were discovered >12 years ago in the nematode Caenorhabditis elegans (15). Comparing human, worm, fruit fly, and plant genome sequences allowed miRNA research to proceed quickly in the past few years. Soon after miRNAs were found in humans, researchers were calculating the number of miRNA genes and focusing on their targets; in 2005 they went on to using them as a signature of cancer in cells and a potential tool for reducing the expression of genes.
MiRNAs are believed to serve fundamental roles in many biological processes through regulation of gene expression by targeting messenger RNAs through translational repression or RNA degradation. Approximately 500 miRNA genes have been identified in the human genome. Many fundamental biological processes are modulated by miRNAs, and an important role for miRNAs in carcinogenesis is emerging. Hence, miRNAs have been shown to regulate oncogenes, tumor suppressors and a number of cancer-related genes controlling cell cycle, apoptosis, cell migration, and angiogenesis (18). Some miRNAs and their target sites were found to be mutated in cancer and miRNA expression profile studies demonstrate that many miRNAs are deregulated in human cancers and are designated as oncogenic miRNAs (OncomiRs). Furthermore, expression patterns of miRNAs are systematically altered in cancers (lymphoma, leukemia, carcinomas, etc.). For example in colon adenocarcinomas, high miR-21 expression is clearly associated with poor survival and poor therapeutic outcome (19). Hence, miRNAs match not only criteria for ideal therapeutic targets because they are causally associated with disease, but also for ideal diagnostic biomarkers because they are easy to measure and have strong associations with clinical outcomes (15). It is predictable that they will have a major importance in theragnostics development in the next years.
Microbial-Based Theragnostics Tests
As reported above, remarkable examples already illustrate the power of theragnostics to target therapy in oncology. Theragnostics will also provide new information about the response of certain patients to specific drugs, and then lead to the development of theragnostic tests, in other key areas such as infectious disease.
Infectious diseases, mainly bacterial infections, are responsible for >17 million deaths worldwide and represent the first cause of death in intensive care units. Adequate antimicrobial therapy is the essential starting point of their therapy. However, use of antimicrobials to treat patients without bacterial infection, as well as the common use of broad-spectrum antimicrobials, is associated with an increasing rate of resistance, which complicates treatment and multiplies healthcare costs. Bacterial identification and antibacterial susceptibility testing methods currently used in clinical microbiology laboratories require at least 2 days because they rely on the growth and isolation of microorganisms. However, the delay of initiation of adequate treatment is a major determinant of success of infectious diseases, underlying the urgent need for rapid and accurate diagnostic tests. Molecular theragnostics tests for infectious diseases are an emerging concept in which molecular biology tools are used to provide rapid (<1 hr), accurate, and more informative diagnostic microbiology assays, thus, enabling better therapeutic interventions and the development of new and more specific antimicrobial agents or futuristic drugs. Two examples illustrate how theragnostics influences infectious diseases diagnosis and treatment.
Theragnostics and Preventive Medicine in Infectiology
Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of nosocomial infection throughout the world. Rates of infection and colonization vary substantially between different hospitals both within and between different countries (4% at admission in intensive care units in the United States). MRSA is a significant contributor to prolonged hospital stay, poor clinical outcome, and increased healthcare costs especially among surgical patients. The major method for instituting control is the microbiological identification of patients either colonized or infected with MRSA, followed by isolation of these patients to prevent cross-infection (20). Screening swabs for the detection and the follow-up of treated carriers of MRSA in high-risk units has been recommended in different guidelines and has been shown to be an effective and cost-avoidant strategy for achieving a sustained decrease of MRSA infections throughout the hospital. MRSA detection has largely relied on conventional culture methods on agar plates, which can take 48–72 hr to obtain a result leading to isolation of the patient for 3–4 days (sometimes more).
In recent years, a number of different molecular methods for the rapid detection of MRSA have been described. Among them, the Infectio Diagnostic Inc-MRSA test (Cepheid, Sunnyvale, CA) is highly specific for detecting MRSA in nasal swabs with a 91.7% sensitivity with a processing time of about 2 hrs (21). Although it is more expensive, IDI-MRSA offers greater detection of MRSA colonization, independent of the swab site, than do conventional selective agars (22). Polymerase chain reaction screening for MRSA with this test at admission to critical care units has been demonstrated to be feasible in routine clinical practice, and to provide quicker results than culture-based screening, leading to a better management of both colonized and infected patients and permitting a significant reduction in subsequent MRSA transmission (23). Hence, assays based on the detection of nucleic acids of microbial agents offer enormous potential for the rapid and accurate diagnosis of infections including detection and identification of the causal microorganism(s), as well as the detection and characterization of genes or mutations associated with antimicrobial resistance and virulence. It may be anticipated that the optimal selection of appropriate antimicrobials by clinicians will be improved gradually as an increasing number of rapid molecular diagnostic tools become commercially available. The maraviroc story is a good example of this “future.”
Theragnostics and New Targeted Antimicrobial Agents
Human immunodeficiency virus (HIV) infects target cells by binding of its envelope gp120 protein to CD4 and a coreceptor on the cell surface. In vivo, the different HIV strains use either CCR5 or CXCR4 as coreceptor. CCR5-using strains are named R5 viruses, whereas CXCR4-using strains are named X4. As X4 viruses usually occur in the later stages, coreceptor usage is used as a marker for disease progression.
Interest in coreceptors increased as a consequence of the development of a new class of antiretroviral drugs, namely the coreceptor antagonists (24). So far, the CXCR4 blockers are not allowed to be used in the clinical practice due to their severe side effects. In 2007, maraviroc, a member of the CCR5 coreceptor antagonists, was approved by the FDA for the treatment of adult patients who are infected with only CCR5-tropic HIV-1 virus, who have evidence of viral replication, and who are resistant to multiple antiretroviral agents (25). Interestingly, both tropism and treatment history should guide the use of maraviroc, which has demonstrated in vitro activity against a wide range of CCR5 tropic clinical isolates, including those resistant to the four currently existing drug classes of antiretroviral agents. In contrast, CXCR4-tropic and dual-tropic HIV-1 entry are not inhibited by maraviroc. Hence, the knowledge of patients’ viral population tropism before the initiation of and during therapy with compounds such as maraviroc may be critical to optimize treatment strategies.
Beside traditional phenotypic assays, there are at least four phenotypic recombinant virus assays available to predict coreceptor usage. The detection of minority variants is a limitation of all population-based assays and varies between 1% and 10%, depending on the assay used. However, recombinant virus assays combine efficiency and accuracy, thus, making them suitable for the clinical management of HIV-infected individuals treated with coreceptor antagonists (24, 26). The two pivotal phase III trials for maraviroc, MOTIVATE-1 and -2, were conducted using the Trofile CCR5 coreceptor assay developed by Monogram Biosciences (South San Francisco, CA) approved for commercial use by the FDA on August 6, 2007. In these studies maraviroc, in combination with optimized background therapy, demonstrated superior virologic and immunologic treatment outcomes over optimized background therapy alone in treatment-experienced patients infected with CCR5-tropic HIV-1. Therefore, the FDA recommends that maraviroc only be used after a test has been conducted to determine that the patient only has virus using the CCR5 coreceptor. The drug should be part of a combination therapy regimen. However, cost is an issue. Whereas the list price for maraviroc is similar to that of second-generation protease inhibitors developed to treat experienced patients ($29 per day), the list price for the assay is $2000.
In addition to targeting drugs to increase efficacy, theragnostics also help to identify patients who might be susceptible to dangerous side effects of medications.
Theragnostics to Increase Drug Safety
In the United States, serious adverse drug reactions (ADR) cause or lead to 6% of all hospitalizations (>2 million hospitalized patients), even when drugs are appropriately prescribed and administrated, and contribute to 100,000 deaths annually (fourth to sixth leading cause of death) (27). Regarding the importance of this problem for healthcare, the FDA has operated the Adverse Event Reporting System since 1998. It collects all reports of ADR submitted directly to the agency or through drug manufacturers. Analysis of this database revealed that, from 1998 through 2005, the number of reported serious and fatal ADR have increased by a factor 2.7 (15,107 deaths in 2005) (28). Thus, the second key application of theragnostics, which consists of identifying the population at risk of aggravated side effects during therapy, is becoming an increasingly important instrument as physicians, patients, regulatory authorities, and payers look for innovative ways to improve the risk/benefit ratio of medicines. Classic causes of the extraordinary variation in patient response to medications are well known, such as age, organ function, and drug interaction. The Human Genome Project opened new opportunities for using genetic information to individualize drug therapy, called pharmacogenetics or pharmacogenomics. Although the terms are sometimes used interchangeably, pharmacogenetics is the study of how inherited DNA variations, typically in just a few genes, affect drug metabolism or toxicity. Hence, depending on the medication, individual genetic variation can account for as much as 95% of variability in drug disposition and effects (27). Pharmacogenomics is a broader term that covers all the technologies that can be used in high-throughput screening (29).
Considerable research has focused on understanding the molecular mechanisms behind ADRs during drug development and clinical practice. Therefore, current analyses estimate the cost of bringing a new drug to market at around $880–$1000 million over 15 years. The goal of integrating pharmacogenomics into the research and development process is to make this process more efficient, and to come up with better drugs not only to help in decision making but also to avoid withdrawal of the drug postmarketing due to an emerging side effect which has been predicted. The National Institutes of Health, through the Pharmacogenetics Research Network, is promoting the discovery of new genes that affect drug metabolism, as well as stronger correlations between known polymorphisms and drug effects. The search for pharmacogenomic biomarkers focuses mainly on genes encoding drug-metabolizing enzymes, whose mutations can lead either to elevated levels of the drug itself and/or of its reactive metabolites or to ineffective underdosing. In contrast, for immune-mediated toxic effects, convincing data are actually restricted mainly on genes of the major histocompatibility complex class I.
Variations of Drug-Metabolizing Enzymes Genes
It is only the dose that makes a drug a poison! (Paracelsus 1493–1541).
The right dosage regimen for an individual patient is one that provides an acceptable balance of benefits and risk of adverse effects. However, traditional approaches in drug development and clinical practice to getting this right dose vary greatly with the therapeutic area of the drug and with the drug’s benefit/risk ratio. Most of the dosing paradigms are simple and easy for physicians and patients to understand and respect the principle of population-acceptable safety and efficacy. This is often referred to as the one-dose-fits-all concept of dosing. In current practice, there are only a few medications where doses are adjusted a priori for patient characteristics known to, or suspected to, change the exposure profile of the drug (e.g., age, renal function, body mass). Some may be also adjusted a posteriori based on response. Too often, however, for drugs with a narrow therapeutic index, the one-dose-fits-all dosing paradigm is not precise enough and new approaches are needed to define the right dose to avoid or minimize ADRs.
Warfarin is a perfect example of this need for a priori strategies to individualize dose. It has been more than a half century since the FDA-approved warfarin as an oral anticoagulant. Since then, warfarin has become the most prescribed anticoagulant worldwide with >23 million prescriptions in 2003 in the United States alone. Despite this marketing success and decades of experience, the safety of warfarin is still an area of intense scrutiny and controversy because the rate of minor and major adverse events from this well-known drug is still among the highest of all commonly prescribed outpatient drugs in the world (30).
The principal challenge for prescribers wanting to administer warfarin is, on one hand, the biological heterogeneity and comorbidity of their patients and, on the other hand, a very narrow therapeutic index. As a consequence, there is greater than ten-fold interindividual variability in the dose required to attain a therapeutic response (30). For this reason, initial dosing of warfarin begins as a “trial and error” exercise with frequent a posteriori adjustments of dose based on the individual observed anticoagulant response as measured by the international normalized ratio.
With the explosion of knowledge on genetic biomarkers related to the pharmacokinetics and pharmacodynamics of warfarin, clinical pharmacologists can now estimate the therapeutic warfarin dose by genotyping patients for single nucleotide polymorphisms that affect warfarin metabolism or sensitivity. Hence, pharmacogenetic analysis of two genes, the warfarin metabolic enzyme, the cytochrome CYP2C9 and warfarin target enzyme, vitamin K epoxide reductase complex 1 VKORC1, confirmed that the variants of this two genes account for 30%–50% of the variability in dosing of warfarin. Studies have shown that patients carrying the allelic variants CYP2C9*2 and/or CYP2C9*3 variants or the common VKORC1 haplotype A/A (or haplotype*2) are warfarin sensitive and typically require lower warfarin doses to reach a therapeutic international normalized ratio with a minimal risk of bleeding (31).
A large recent study confirmed that the presence of each variant is a significant predictor of the time to the first international normalized ratio of >4. Only the VKORC1 haplotype A/A was associated with a decreased time to the first international normalized ratio within the therapeutic range. This result supports the decision of the FDA in August 2007 to update the label of warfarin to include information on pharmacogenetic testing and to encourage, but not require, the use of this information in dosing individual patients initiating warfarin therapy. Pharmacogenetic-guided dosing of warfarin is a promising application of “personalized medicine.” Two recent trials of genotype-guided vs. standard warfarin therapy have recently confirmed the feasibility and the benefits of this approach (32, 33). Hence, algorithms incorporating genetic (CYP2C9 and VKORC1), demographic, and clinical factors to estimate the warfarin dosage, could potentially minimize the risk of overdose during warfarin induction.
Similar examples of genetic variants responsible for serious ADRs have been described in oncology. The chemotherapy drug irinotecan is one of the most important drugs in advanced colorectal cancers despite its sometimes unpredictable adverse effects (diarrhea and neutropenia). To understand why some patients experience severe adverse effects whereas others do not, phamacogenetic analysis of metabolic pathways was conducted and suggested that the drug was particularly toxic in a subset of patients homozygous for UGTIA1*28, a variant sequence in a promoter region of the UGTIA1 gene, that codes for the bilirubin detoxifying enzyme, UDP-glucuronosyltransferase. About 10% of patients are homozygous for the genetic variant, which boosts their chances of developing a dangerously low level of white blood cells, a known side effect of the drug (34). As a consequence, the FDA has revised the package insert of irinotecan to warn of the association between toxicity and UGTIA1*28.
In addition to UGT1A1, thiopurine methyltransferase, which metabolizes the anticancer drug 6-mercaptopurine, is also a well-known candidate for current theragnostics application, because thiopurine methyltransferase-deficient patients are at risk of severe bone marrow toxicity at otherwise normal drug dosages. Physicians now routinely prescreen children with leukemia with the thiopurine methyltransferase test that aims to predict the risk of severe neutropenia for the purine drugs azathioprine and 6-mercaptopurine at relatively low cost (35). As for irinotecan, the FDA decided that evidence indicates sufficient benefit to warrant informing prescribers, pharmacists, and patients of the availability of pharmacogenetic tests and their possible role in the selection and dosing of 6-mercaptopurine and has approved label changes for this drug to include pharmacogenetic testing as a potential means to reduce the rate of severe toxic events.
In December 2004, the FDA approved the Roche Molecular Systems’ AmpliChip Cytochrome P450 (CYP450) test, an array that detects 29 variations in two genes, CYP2D6 and two in CYP2C19. These enzymes play key roles in drug metabolism, and genetic variations in the genes can affect the rate of drug metabolism. The product, which is now available, could have major implications for dosing and choice of therapy for a variety of pharmaceuticals, including cardiovascular drugs and antidepressants. Although cytochrome P450 is important, it is not the only gene to affect drug response. As many as 180 genes could affect drug metabolism, including metabolic enzymes, transporters, and other proteins. By this count those genes contain at least 2000 different variants, and a truly comprehensive metabolic genotyping panel, would have to test for all of them The Affymetrix drug-metabolizing enzymes and transporters (Santa Clara, CA) early access provides a solution to this challenge, by combining molecular inversion probe technology with universal microarrays. The drug-metabolizing enzymes and transporters solution profiles 1069 drug metabolism biomarkers and automatically interprets data into a commonly used format that can be integrated into clinical trial workflows (36).
Hypersensitivity Reactions and Variations of Major Histocompatibility Complex Class I Genes
Among the validated pharmacogenetics biomarkers, some human leukocyte antigen allelic variants have been consistently associated with immune-mediated toxic effects of medications from different classes, such as carbamazepine, ximelagatran, or abacavir. Abacavir is a nucleoside reverse transcriptase inhibitor that has been prescribed to almost 1 million HIV patients during the past decade and which is used in combination with other medications to treat HIV infection (37). In white populations, hypersensitivity reaction (HSR) occurs within the first 6 weeks of therapy in 5%–8% of patients receiving this drug. Signs and symptoms of abacavir HSR are nonspecific (fever, rash, vomiting, diarrhea) making the diagnosis challenging, particularly in medically complex patients. Clinical management is aimed at supportive therapy and discontinuation of abacavir. Rechallenge with abacavir is contraindicated due to the risk of precipitating a life-threatening reaction. Since 2002, several small studies demonstrated that identification of patients at risk of developing abacavir hypersensitivity through routine genetic screening for human leukocyte antigen HLA-B*5701 led to a drastic reduction of HSR associated with abacavir, representing a significant advance in the field of pharmacogenomics, with an apparent 100% negative predictive value. Recently, these results have been confirmed in a very large double-blind prospective study which showed HLA-B*5701 screening reduced the risk of HSR to abacavir, confirming that a pharmacogenetic test can be used to prevent a specific toxic effect of a drug (38). Even in white populations, 94% of patients do not carry the HLA-B*5701 allele; all data suggest that pharmacogenetic screening for HLA-B*5701 before abacavir prescription is cost-effective (38, 39).
Theragnostics to Monitor Treatment Response
The third key application of theragnostics is to monitor the drug efficacy to adjust the most effective and nontoxic drug dosage. Recent advances in the understanding of the molecular pathways of disease have resulted in a method to design medical diagnostics that can be used to monitor therapy. As previously, most theragnostic applications for drug monitoring are focused on the field of antiviral drugs, as illustrated by viral load measurement which indicates the effectiveness of HIV antiretroviral treatment, and oncology, as shown by theragnostics imaging and monitoring of new cancer drugs.
Theragnostics Imaging in Oncology
As reported previously in this review, anticancer drug development is a major area of research that has received a lot of attention from theragnostics to improve both drug targeting and safety. Imaging monitoring of response to new anticancer drugs has undergone an evolution (and a revolution) from structural imaging modalities to targeting functional metabolic activity at cellular level, to better define responsive and nonresponsive cancerous tissues. Positron emission tomography (PET) has already made a major contribution in this progress and it may be predicted that the development of novel PET tracers and improvements in technology will continue to augment the potential of PET and enhance its attractiveness as an instrument to facilitate drug development (40).
PET is a noninvasive functional imaging technology that provides rapid, reproducible, in vivo assessment, and quantification of several key biological processes important in cancer development and progression that are targeted by anticancer therapies. Previously, estimation of responses to new biological drugs used inaccurate measures of efficacy, such as changes in tumor size or serial invasive testing by tumor biopsies. PET provides information complementary to conventional anatomical imaging, demonstrating utility in a range of cancer settings from diagnosis, tumor stratification, and staging (41).
The traditional example is PET imaging with the biomarker F-18 fluorodeoxyglucose, reflecting tumor glucose metabolism, which offers relevant unique information regarding treatment response. Several mechanisms may influence the enhanced glucose uptake in cancer cells, including up-regulation of glucose transporters, increase the activities of hexokinase and Akt, which seem to play a key role in the control of glucose metabolism together with proteins involved in the signaling pathway, such as mTOR, the target of rapamycin. Furthermore, it has been shown that changes in tumor glucose metabolism precede changes in tumor size underlying the sensibility of the method and reflect directly drug effects at a cellular level (high specificity). Thus, it has been demonstrated in patients with gastrointestinal stromal tumors and other sarcomas who received treatment with imatinib that PET with F-18 fluorodeoxyglucose is appropriate for treatment monitoring (42). F-18 fluorodeoxyglucose PET not only enables the prediction of therapy response early in the course but also determines the viability of residual masses after completion of treatment.
PET has also been used to examine other drug-induced metabolism modification, cellular proliferation, and tissue perfusion (41). Also changes induced by immunomodulating drugs, such as aptosis, telomere activity, and growth factor levels can be studied using specific radiolabeled-PET tracers, whereas conventional imaging modalities may not prove useful in such scenario (43).
Theragnostics imaging like PET is also very useful in mAbs therapy. MAbs have been approved for use as diagnostics and therapeutics in a broad range of medical indications, but especially in oncology. Immuno-PET uses PET technology to track and quantify mAbs in vivo to monitor mAb-based therapy. The availability of proper positron emitters, sophisticated radiochemistry, and advanced PET-computed tomography scanners has been crucial in these developments. For example, PET pharmacokinetic studies will allow a rapid assessment of novel mAbs biodistribution before decisions on whether or not to proceed with the development of the new drug are made. Immuno-PET will play an important future role in the improvement and tailoring of therapy with existing mAbs, and in the efficient development of novel mAbs.
Monitoring of New Cancer Drugs
A growing number of patients with advanced NSCLC require second line treatment after progression or relapse after frontline therapy. Gefitinib is a selective inhibitor of the EGFR tyrosine kinase, which is overexpressed in many cancers, including relapsing NSCLC. If different genetic biomarkers (germline polymorphisms in EGFR gene and high-EGFR copy number) are associated with gefitinib efficacy, their role in predicting drug response is still under examination because of conflicting results in different studies. Furthermore in clinical practice, the use of specific cancer biomarkers necessary for genetic biomarker testing is hampered by the lack of sufficient tumor tissue especially for lung cancer patients. However, it has been reported that drug action can be monitored in available surrogate tissue, such as buccal mucosa and epithelial or normal skin (44). Thus, epithelial cells obtained from buccal mucosa of patients with an objective response to gefitinib presented a reduction in expression of p-EGFR, p-Mapk, and p-Akt after gefitinib treatment, highlighting the in vivo efficacy of this tyrosine kinase inhibitor, whereas the mucosa epithelial cells from patients with progressive (drug resistant) disease had increased or unchanged phosphoproteome. Unfortunately, some NSCLC nonresponders still had evidence of EGFR downstream signaling inhibition with gefitinib, suggesting that redundant signaling pathways might regulate the growth and/or survival of NSCLC.
Perspectives and Conclusions
Blockbuster Versus Targeted Business Model for New Drug Research and Development.
For the past 30 yrs, large pharmaceutical companies have maintained an unchanging core focus on the blockbuster model of drug development (typically defined as products with annual revenues in excess of $1 billion). The success of blockbuster drugs compensated for the numerous molecules that unpredictably did not make it out of phase 1 or 2 clinical testing. In the past few years, this model has come under intense pressure on several fronts. First, the absolute need to prove clinical efficacy and cost-effectiveness of new medicines has led to an expanding cost and decreasing productivity of research and development (2). Meanwhile, reimbursement is getting tougher as insurers become increasingly reluctant to pay the high price of innovative drugs and threaten to limit reimbursement levels. Finally, the switch to generic drugs is becoming routine once a blockbuster drug loses its patent protection. Theragnostics that fuses therapeutics and diagnostic medicine has the potential to positively impact these challenges and may change the classic business model. Through pharmacogenetics and pharmacogenomics, this strategy has capability to lead to improved productivity by using molecular biomarkers to enrich clinical trials with known responders, to exclude those at risk for serious adverse events, and to individualize dosing to genetic profiles of drug. Theragnostics has also the potential to reduce risk and costs, potentially speeding market admission and, ultimately, enhancing the commercial success of the medicine by two mechanisms. First, theragnostics’ ability to identify appropriate patients and measure efficacy with objective diagnostic criteria has a huge potential to increase sales. Second, the life of the new drug is expanded, because the medication is tightly linked to a diagnostic test which has patent protection beyond the expiry of the medicine patent.
Even if narrowing the market by targeting a subpopulation seems to be highly undesirable from a business perspective, the current commercial imperative is to lower costs by improving the clinical development success rate. Thus, nontargeted therapies that are marginally effective may turn out to have a substantial benefit when paired with an appropriate theragnostics test. Similarly, theragnostics may make it possible to explore, in the same series of trials, the efficacy of targeted and nontargeted approaches. Such development programs could salvage some new drugs and improve overall success. This new model is strongly supported by the FDA, which released a voluntary genomic data guidance meant to assist both regulatory agencies and pharmaceutical companies in evaluating the potential benefit of implementing theragnostics tests during the preclinical and clinical phases of drug development (http://www.fda.gov/cder/genomics/FDAEMEA.pdf).
Theragnostics and Personalized Medicine: A Political Issue
In August 4, 2006, Senator Barack Obama introduced the bill entitled “Genomics and Personalized Medicine Act of 2006” (S.3822) that mandates technology convergence between pharmaceuticals and diagnostics with the stated intention to “improve access to and appropriate utilization of valid, reliable, and accurate molecular genetic tests by all populations, thus, helping to secure the promise of personalized medicine for all Americans.” In particular, Senator Obama’s bill recognizes the enormous potential of pharmacogenomics to “better target the delivery of healthcare, facilitate the discovery and clinical testing of new products, and help determine a patient’s predisposition to a particular disease or condition.” Ultimately, this would “increase the efficacy and safety of drugs and reduce healthcare costs” (http://www.theorator.com/bills109/s3822.html).
To achieve these aims, very innovative measures are proposed:
- Creation of a “Genomics and Personalized Medicine Interagency Working Group” within the Department of Health and Human Services.
- Incentive actions to encourage genomics and biobanking research.
- Enhancement of genomics workforce training.
- An income tax credit for patients paying for genetic tests or the research related to that test.
This legislation has the potential to provide significant incentives to the industry to expand and accelerate pharmacogenomic research efforts, with the intent of speeding up the approval process for new drugs and providing additional companion diagnostic tests. Finally, this legislation may force current industry players into new and unexpected alliances, thereby reshaping the process in which pharmacogenomic products are brought to market. Hence, theragnostics and personalized medicine in the future might depend on the results of the United States presidential election of 2008.
In summary, it is increasingly clear that the development of new technologies, both genetic and nongenetic, offer new opportunities for a better understanding of the underlying mechanisms of disease. These advances will facilitate the development of new classes of targeted medicines as well as sensitive and specific diagnostic tests. It is highly likely that the maximum value for these advances will be gained where the diagnostic and therapeutic applications of this knowledge are bought together in the developing field of theragnostics.
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