Nanocarriers for Cancer Diagnosis and Targeted Chemotherapy

By Muhammad Raza Shah, Muhammad Imran and Shafi Ullah

Nanocarriers for Cancer Diagnosis and Targeted Chemotherapy reviews the principles and applications of nanocarriers for targeted drug delivery. Drug targeting involves active and passive strategies that exploit both the use of ligands for interactions and the physical and chemical properties of nanocarriers and micro-environments at target sites. Multidrug resistance and adverse side effects associated with anticancer drugs have attracted greater scientific attention and led formulation scientists to specifically target these drugs to target sites. Nanocarriers like liposomes, niosomes, gold nanorods, carbon nanotubes, and micelles are discussed for the delivery of drugs to specific disease sites.

This is an important reference source for researchers in the biomedical and biomaterials fields who want to gain an understanding on how nanotechnology is used for earlier diagnoses and more effective cancer treatment.

• Explores the fundamental principles of drug targeting through different nano-carriers, highlighting major applications
• Shows how the use of nanocarriers is leading to quicker cancer diagnosis and more effective treatment
• Discusses the major challenges of using nanocarriers for drug delivery and assesses how to overcome these barriers

• Language: English
• Publisher: Elsevier Science
• Release date: Jul 13, 2019
• ISBN: 9780128163115

Muhammad Raza Shah

Muhammad Raza Shah is a full professor at the International Center for Chemical and Biological Sciences, HEJ Research Institute of Chemistry, University of Karachi, Pakistan. He is also the Head of the Center for Bioequivalence Studies and Clinical Research. He is a recipient of several awards, including the Tamgha-i-Imtiaz Award from the President of Pakistan, the Salam Prize, the Professor Atta ur Rahman Gold Medal, and the Dr M Raziuddin Siddiqi Prize, by the Pakistan Academy of Sciences, for scientists under 40 years of age, in the field of chemistry. Professor Shah has authored six books and edited four books, in addition to contributing over 350 peer-reviewed journal papers. One of his authored books was declared as best book of 2017 by the Government of Pakistan’s Higher Education Commission.

Book preview

Preface

Muhammad Raza Shah, International Center for Chemical and Biological Sciences, H.E.J. Research Institute of Chemistry, University of Karachi, Karachi, Pakistan

Cancer causes significant reduction in a patient’s quality of life and increases the economic burden and mental stress of families and societies. Multiple treatment procedures may be adopted to treat cancer. After diagnosing cancer and determining its stage, a treatment strategy is normally devised by doctors. There are three general treatment methods for cancer: chemotherapy, radiation therapy, and surgery. Chemotherapy involves administration of anticancer drugs to patients, inhibiting the growth of rapidly growing cancerous cells. It is considered to be the method of choice for the treatment of cancer due to problems associated with surgery and radiotherapy. Currently, anticancer drugs have low aqueous solubility and biological membrane permeability, nonselectivity towards cancerous cells, and rapid clearance from the body. In addition, the development of multidrug resistance reduces chemotherapy’s positive clinical outcomes and increases off-target side effects.

Cancer treatment with chemotherapy can be improved through selective delivery of anticancer drugs to tumors in increased concentrations, thus enhancing their therapeutic efficacy with minimal side effects. Various nanotechnological approaches are used for increasing the aqueous solubility, membrane permeability, retention time in the biological system, and specific delivery to target sites of anticancer drugs. These approaches exploit the design of nanocarriers that are capable of performing a wide variety of functions including drug delivery, diagnosis, and monitoring of treatment strategies.

The book Nanocarriers for Cancer Diagnosis and Targeted Chemotherapy covers selected topics related to the targeted delivery of chemotherapeutic agents for diagnosis and treatment of various types of cancers. The nanocarriers discussed in this book range from liposomes, niosomes, carbon nanotubes, and stimulus-responsive micelles to nanoradiopharmaceuticals and metal nanoparticles. The book also covers various physical and biological barriers that reduce the clinical efficacy of anticancer drugs. Nanotechnological strategies used for selective delivery of anticancer drugs to cancerous tissues are also discussed in detail. Most important, the authors elaborate on the use of surface-engineered nanocarriers for effective diagnosis and treatment of cancers of vital organs such as the brain, liver, and lungs. The book also discusses the use of multifunctional nanocarriers for improved tumor localization and retention of radionuclides and simultaneous radiotherapy, chemotherapy, diagnosis, and monitoring.

The authors of this book have a good track record of publishing top-quality research articles in leading international journals related to drug delivery and nanotechnology. The book will be useful for industry as well as for academic researchers, libraries, and consultants. It also provides a wealth of information for undergraduate and graduate students in the field of drug delivery. Finally, the book will prove to be an updated source of information and useful supplementary reading for both teachers and students.

Chapter 1

Potential physical and biological barriers leading to failure of cancer chemotherapy

Abstract

Cancer is among the deadliest diseases in the world, with millions of new cases reported each year. Chemotherapy is preferred for cancer treatment due to the invasive nature of and other problems associated with surgery, laser therapy, and photodynamic therapy. Currently used chemotherapeutic agents face various issues that limit their effective use for cancer treatment. These issues arise either from the physicochemical properties or biological barriers associated with the tumor microenvironment or biological systems. The physicochemical barriers are associated with the anticancer drugs and include their lower aqueous solubility and membrane permeability, nonselectivity toward cancerous cells, and instability in the intestine due to harsh acidic and enzymatic actions. The biological barriers that reduce the therapeutic efficacy of anticancer drugs include transmembrane drugs’ efflux transporter proteins, presystemic metabolism, nonlinear pharmacokinetics, hepatic impairment and malabsorption, genetic variations, physiological characteristics of tumor tissues and their unique microenvironment, and impediments erected by the blood–brain barrier for successful treatment of brain tumors. This chapter covers both the physicochemical and biological barriers that reduce the therapeutic efficacy of chemotherapeutic agents used for the treatment of various types of cancers.

Keywords

Cancer; chemotherapy; barriers; physicochemical; biological

1.1 Introduction

Cancer is a group of disease where abnormal cell growth and a failure to control it lead to tumor growth that can spread and invade other parts of the body. Cancer starts from an uncontrolled cellular growth to form a heterogeneous primary tumor with premature and leaky vasculature followed by spread or metastasis of the cancer cells to other parts of body to form secondary tumors. The pathogenesis of cancer is very dynamic as it involves constantly altered cellular and molecular interactions in different pathways (Hanahan and Weinberg, 2011).

Cancer has emerged as a major health issue all over the world. An alarming increase in cancer incidence has been predicated by global demographic characteristics for the coming decades, with 420 million new cancer cases annually expected by 2025. Lung, prostate, female breast, and colorectal cancers are the most frequently diagnosed cancers in Europe. Among all the types of cancer, lung cancer is the greatest cause of cancer incidence and mortality around the globe (Ferlay et al., 2013, 2015). People of all societies around the world, women and men, the young and old, the poor and rich, have been badly affected by cancer, which increases the economic burden and mental stress on families, patients, and societies. A substantial number of cancer patients experience a significant reduction in their quality of life due to economic hardship, physical pain, and mental anguish (Xu et al., 2016).

Host–environment interactions and persistent tissue injury are the leading causes of cancer occurrence. Moreover, exposure of carcinogenic agents such as ultraviolet radiations, tobacco, and infections for a longer time results in different epigenetic, genetic, and global transcriptome changes through inflammatory pathways that in turn lead to increased cancer risk and cancer incidence (Hidalgo et al., 2010). The pathogenesis of cancer is very dynamic as it involves constantly altered cellular and molecular interactions in different pathways. Six hallmarks of cancer have been established that can distinguish characteristics between the tumor and normal tissues and may provide better alternative treatments: sustained singling for proliferation, growth suppressors escaping, activation of invasion and metastasis, replicative immortality, induction of angiogenesis, and development of resistance to cell death (Orgogozo et al., 2015). Maintenance of growth signals and continuous proliferation of cancer cells are the prominent characteristics of almost all types of cancers. Similarly, the inhibition of cells’ normal regulators and apoptosis is also associated with cancers. The development of higher resistance in the cells to apoptosis helps the cancer cells to maintain their DNA integrity and thus to keep replicating continuously. Cancer cells get nutrients and dispose their wastes through angiogenesis, the formation of new blood vessels. New or secondary tumors are formed as the cancer cells migrate to new sites. Evasion from immune destruction and establishment of energy metabolism are the two emerging hallmarks of cancer cells. Cancer cells upregulate glucose transporter expression, thus reprogramming their metabolic pathway to aerobic glycolysis. The resultant metabolic switch in turn leads to the generation of amino acids and nucleosides, which facilitate additional growth and proliferation. Another important aspect of cancer cells is that T-lymphocytes markers that help in recognition and destruction of abnormal or foreign cells do not get well expressed on cancer cells. This allows the cancerous cells to avoid elimination by the immune system (Jogi et al., 2018; Orgogozo et al., 2015).

Early diagnosis of cancer has been an important point for the treatment of cancer. It has been very difficult to diagnose cancer during early stages as its clinical symptoms show up very often before cancer progresses to an incurable stage. Various treatment strategies are used alone or in combination for achieving effective clinical outcomes and early recovery. Surgery is one of the treatment strategies for the treatment and management of cancer. Surgery is selected in case the tumor is benign and its complete removal is easy and is not associated with any harm other vital and healthy organs of the body (Perros et al., 2014). Surgical removal of the tumor is associated with certain limitations such as infections at the sites of surgery that can harm the normal tissues and can lead to morbidity and mortality. Moreover, occurrence of surgery associated infections also increases the treatment cost (Johnson et al., 2016). Moreover, the surgery failure can cause recurrence of cancer and metastasis which in turn decreases patient life expectancy by half (Lukianova-Hleb et al., 2016).

Radiotherapy involves the use of radiations and destroys cells by damaging their components. Radiotherapy is preferred for treatment of a local tumor in combination with surgery (Allen et al., 2017; Fournier et al., 2016). It is used when surgery cannot remove the tumor completely and chances of normal tissue damage exist. However, radiotherapy damages healthy cells along with cancerous cells in addition to being time consuming and quite expensive (Tekade et al., 2014). Side effects of radiotherapy include irritation and damage to skin upon repeated exposure to radiation beams. Damage to salivary glands and hair loss most frequently occur in cases of neck and head cancer treatment with radiotherapy (Huang et al., 2015). Immunotherapy involves inducing, enhancing, or suppressing the immune system using various immunological agents (De Vries and Figdor, 2016). It is associated with risks such as swelling, itching, soreness, and redness in the area of administration. Immunological changes can also result in weight gain owing to extra fluids, diarrhea, heart palpitations, and infections. Immunotherapy also affects other vital organs and is considered time consuming (Gotwals et al., 2017). Laser therapy involves penetrating the tumor with laser light and destroying it through increase in temperature. It also causes the tumor to shrink and deprives it of oxygen and nutrition (Rhee et al., 2016). Photodynamic therapy involves a photosensitizing agent that is activated through a specific type of light and kills cancerous cells through a chemical reaction. Cervical, skin, rectum, and colon cancers are treated with laser light. Laser light is also used for the removal of small growths that can result in the development of cancers. Shortcomings associated with laser light include the need to repeat laser light applications over a prolonged period for complete removal of the tumors (Sonis et al., 2016).

Owing to the invasive nature and other problems associated with these cancer treatments, preference has been given to chemotherapy for cancer treatment and management. Chemotherapy involves the systemic administration of anticancer drugs to patients for inhibiting the growth of rapidly growing cancerous cells (Jabir et al., 2012). Rapid division is the main characteristic of cancerous cells destroyed by conventional anticancer drugs given as part of chemotherapy strategy. The combination of chemotherapy with any other treatment strategy is currently preferred for the management and treatment of cancer due to its increased, quick, and positive responses; decreased resistance; and higher tolerability (Xu et al., 2015). Unfortunately, expected clinical outcomes of chemotherapy cannot be achieved due to certain barriers. Physicochemical properties of anticancer drugs such as lower aqueous solubility and permeability result in their inferior clinical outcomes (Thanki et al., 2013). To exert pharmacological action, a sufficient and reproducible amount of a drug should reach the target site, a process that depends on the solubility of the drug in plasma and other physiological fluids (Bhattachar et al., 2006). For a drug to be absorbed, it first has to dissolve in the physiological fluids (Bhattachar et al., 2006). Thus the lower aqueous solubility of most anticancer drugs reduces their therapeutic efficacy, making expected clinical outcomes unachievable. Once a drug has been solubilized, it has to pass through a biological barrier to reach the receptors or site of action. Drug permeability thereby influences the overall pharmacokinetic behavior of the drug. Poor biological membrane permeability of anticancer drugs reduces their biological performance and ultimately their therapeutic efficacy (Shaikh et al., 2012; Sugano, 2011).

Currently used chemotherapeutic agents do not differentiate between normal and cancerous cells and are nonspecific in their action toward cancer cells. This in turn results in side effects on normal cells including organ dysfunction, alopecia, mucositis, and even thrombocytopenia or anemia. Such side effects lead to inferior clinical outcomes of the chemotherapy due to treatment delay, dose reduction, or discontinuance of anticancer drugs (Nguyen, 2011; Coates et al., 1983). Another major barrier decreasing the efficacy of anticancer drugs is their early clearance from the systemic circulation. They are washed from the body due to being engulfed by macrophages. Thus they remain in systemic circulation for a brief period of time and cannot interact with the cancerous cells, resulting in lower therapeutic effects (Jabir et al., 2012). Moreover, a multidrug resistance protein, P-glycoprotein, is overexpressed on cancerous cells surfaces. It acts as an efflux pump that prevents the drug from accumulating inside the tumors, resulting in mediation of resistance to anticancer drugs. This ultimately leads to the failure of chemotherapy (Davis and Shin, 2008; Sutradhar and Amin, 2014). In case of solid tumors, cell division may effectively cease near the center, making chemotherapeutic agents insensitive to chemotherapy. Furthermore, the killing of cancerous cells cannot be achieved effectively with anticancer drugs due to their lower penetration into the core of the solid tumors (Tannock et al., 2002).

The barriers resulting in the poor therapeutic efficacy of anticancer drugs can be broadly divided into two categories: physicochemical and biological. This chapter aims to discuss both categories of the barriers in detail. The underlying mechanisms will be highlighted with examples.

1.2 Physicochemical barriers resulting in reduced therapeutic efficacy of anticancer drugs

The physicochemical properties of anticancer drugs, such as lower aqueous solubility and biological membrane permeability, nonselectivity toward cancer cells, and enzymatic degradation, greatly reduce the efficacy of chemotherapy for treating and managing various types of cancers. This section describes the physicochemical properties of anticancer drugs that act as barriers for achieving effective results of chemotherapy.

1.2.1 Poor aqueous solubility of anticancer drugs

Bioavailability refers to the rate and extent of drugs that reach systemic circulation for displaying their intended use after their absorption. The therapeutic responses of the drugs can only be achieved when they become bioavailable in systemic circulation (Thanki et al., 2013). Solubility can be defined as the property of a solid, liquid, or gaseous substance or solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution. The solubility extent of a solute in a solvent is defined as a level of saturation concentration that cannot be increased by adding more solute in the solution (Lachman et al., 1976).

Decreased aqueous solubility of drug molecules has long impeded their therapeutic efficacy. The Biopharmaceutical Classification System (BCS) classifies drugs as having poor aqueous solubility if their highest doses are not soluble in 250 mL or less of aqueous media over a pH 1–7.5 range (Kipp, 2004). Less water-soluble drugs are required to be taken in larger doses to achieve therapeutic plasma concentrations after oral administration. Lower aqueous solubility is one of the major issues encountered with the formulation development of both currently available drugs and new active chemical entities. Solubility of drugs in an aqueous medium is the basic requirement for their absorption.

Water has been the solvent of choice for designing pharmaceutical dosage form for drugs. But because anticancer drugs such as etoposide, paclitaxel, and docetaxel are weakly basic or acidic, they demonstrate poor solubility in water (Savjani et al., 2012). They are chemically complex molecules containing bulky polycyclic moieties. This results in high lattice energy for their dissolution, making them unable to form hydrogen bonds with water (Lukyanov and Torchilin, 2004; Narvekar et al., 2014). Oral delivery of such anticancer drugs is limited as improved bioavailability of the drugs cannot be achieved. The BCS places these drugs in class II as needing their aqueous solubility enhanced for improved bioavailability and therapeutic efficacy. Rubitecan, tamoxifen, sorafenib, and gefitinib are classic examples of drugs in this class (Thanki et al., 2013). The poor aqueous solubility of the drugs leads to more intensified issues in case of their intravenous (IV) infusion, which is often preferred for enhanced pharmacokinetics and reduced gastrointestinal toxicity. But lower water solubility limits IV administration (Narvekar et al., 2014). Anticancer drugs having lower aqueous solubility are cleared from the bloodstream before reaching tumor tissues (Tran et al., 2017).

1.2.2 Lower membrane permeability of anticancer drugs

Lower permeability also reduces the efficacy of anticancer drugs. Drug molecules reach the systemic circulation by penetrating the intestinal epithelium. This happens in three different ways: through carrier-mediated active transport, concentration-dependent passive diffusion, and specialized routes such as paracellular transport and endocytosis. Most drug molecules are absorbed through passive diffusion (Pade and Stavchansky, 1998; Masaoka et al., 2006). Because the epithelial plasma membrane contains phospholipids, a drug can only cross it if it possesses a certain level of lipophilicity. Log P value, the octanol/water partition coefficient, predicts the lipophilicity of drug molecules and thus a drug’s permeability across the biological membrane. Higher and lower Log P values result in poor permeation of drug molecules while Log P values in the midrange generally result in high permeability (Skolnik et al., 2010). Apart from Log P, factors such as molecular size, hydrogen bonding ability, molecular weight, and ionization have a profound effect on permeability (Lipinski, 2000). Anticancer drugs with higher aqueous solubility and lower permeability are in BCS class III and include anastrozole, cyclophosphamide, doxorubicin, letrozole, and methotrexate. These drugs cannot be orally delivered as they cannot permeate the biological membrane, so lower oral bioavailability and, ultimately, lower therapeutic efficacy are achieved (Thanki et al., 2013).

1.2.3 Anticancer drug instability in intestine

Because the small intestine has a large surface area due to villous structures and folding of the epithelial, it has been site of absorption for most anticancer drugs. The intestinal epithelium is covered with goblet cells, absorptive cells, and endocrine cells. A pH gradient with pH 2 in the stomach, pH 5–6 in the jejunum, and pH 7–8 in the ileocecal valve occurs due to sections of the cells covering the intestinal epithelium, pancreas, and bile (Evans et al., 1988; Ewe et al., 1999; Press et al., 1998; Wilson and Washington, 1989). The drugs’ absorption depends upon their solubility in the intestinal sections as well as on the length of their transit in the intestine. Solubility of some of the anticancer drugs also depends upon the pH of a section. Acidic anticancer drugs are inversely correlated to the pH of the solution due to their ionization at a high pH. Moreover, some tyrosine kinase inhibitors such as gefitinib, dasatinib, and lapatinib demonstrate dramatically decreased solubility above a certain pH (Eley et al., 2009; Stuurman et al., 2013). Clearly, both the absorption and bioavailability of anticancer drugs are greatly affected by the pH gradient of the intestine. This aligns with how the drugs interact with antacids. The maximum plasma concentration and systemic exposure of dasatinib are decreased by 58% and 55%, respectively, upon simultaneous intake of the drug with antacids (Eley et al., 2009). Both the composition and pH of intestinal fluids affect the stability of anticancer drugs as they degrade (Joel et al., 1995a,b). Anticancer drugs chlorambucil and etoposide show variable and low oral bioavailability due to their deceased chemical stability in gastric and intestinal fluids (Toffoli et al., 2004; Adair et al., 1986).

1.2.4 Anticancer drug nonselectivity toward tumors

Currently used chemotherapeutic agents do not differentiate between normal and cancerous cells and are nonspecific in their action toward the latter. An effective therapeutic dose of anticancer drugs in tumor tissues is achieved at the expense of massively contaminating normal tissues, which can result in side effects such as organ dysfunction, alopecia, mucositis, and even thrombocytopenia or anemia. These effects lead to inferior clinical outcomes of the chemotherapy due to treatment delay, dose reduction, or discontinuance of anticancer drugs (Nguyen, 2011; Coates et al., 1983).

1.3 Biological barriers resulting in reduced therapeutic efficacy of anticancer drugs

Delivery of anticancer drugs into the tumor tissues in efficient concentrations depends on the properties of various biological barriers associated with cancer. This section describes these biological barriers in detail.

1.3.1 Effects of transmembrane anticancer drug efflux

Drugs transmembrane efflux can be defined as the drug molecules’ expulsion across the cellular membrane from the cells through a systematic transportation system such as breast cancer–resistant protein (BCRP), P-gp, multidrug-resistant associated protein (MRP), cytoplasmic transport, methotrexate efflux (folates), fluorochrome efflux (Buxton, 2006; Hunter and Hirst, 1997; Breedveld et al., 2006). ATP-binding cassette (ABC) transporters are transmembrane proteins that transport different substrates across intracellular and extra membranes. Most chemotherapeutic agents are the substrate of active-drug efflux ABC transporters that get overexpressed on the epithelial layer of the gut wall. Moreover, the transporter proteins are also expressed in tumor tissues and other tissues with an excretory function such as the placenta, the epithelial layer of renal tubules, the biliary canalicular membrane of hepatocytes, the blood–brain barrier (BBB), and the blood–testis barrier for active clearance (Schinkel and Jonker, 2012).

P-gp is one of the important efflux transporters that leads to drug molecules being excreted back into the intestinal lumen. It is encoded by the multidrug resistance-1 gene and is localized in enterocytes. It is widely distributed in different glands, hepatocytes, intestinal epithelia, kidneys, and capillary endothelial cells that comprise the blood–testis and BBBs. P-gp is membrane-associated protein with N terminal glycosylation and a molecular weight of 170 kDa. It is classified as a member of the superfamily of ABC transporters. It contains two homologous chains of similar length, each consisting of six units of transmembrane domains and two ATP-binding sites separated by a flexible linker polypeptide region between the two homologous chains (Ambudkar et al., 1999). The P-gp operation takes place in the three major areas: the luminal (apical) membrane enterocyte, where it limits the drug entering into the body; the canalicular membrane of hepatocytes, where it increases the drug’s elimination into urine and bile; and various sensitive tissues such as lymphocytes, brain, testis, and fetal circulation, where it limits the drugs penetration (Fromm, 2004). When P-gp gets expressed in the apical membrane of the epithelial layer of the gut wall, it prevents the uptake of many oral anticancer amphiphilic drugs including irinotecan, topotecan, docetaxel, paclitaxel, idarubicin, etoposide, vinorelbine, doxorubicin, and almost all tyrosine kinase inhibitors (Ni et al., 2011; Dohse et al., 2010; Bansal et al., 2009; Bardelmeijer et al., 2002; Lacayo et al., 2003; Banna et al., 2010).

The MRP family basically transports anionic hydrophobic conjugates and extrudes uncharged hydrophobic drug molecules. Because the apical site of the epithelial membrane contains MRP2 and MRP4, it contributes significantly to the efflux of anticancer drugs (Kuppens et al., 2005). Tissues of renal tubules, epithelium of the intestines, placenta, and hepatocytes overexpress MRP2. Anticancer drugs such as docetaxel, etoposide, mitoxantrone, vinblastine, paclitaxel, methotrexate, doxorubicin, epirubicin, irinotecan, vincristine, and cisplatin are the major substrates of MRP2 transporter enzymes (Kuppens et al., 2005; Chan et al., 2004; Suzuki and Sugiyama, 2002; Huisman et al., 2005). Similarly, MRP4 gets expressed in tissues with excreting functions at a level of expression much lower than those of P-gp and MRP2. This results in the limited role of MRP4 in anticancer drugs’ efflux transport (Chan et al., 2004). Chemotherapeutic agents that are substrates for MRP4 include purine analogs and methotrexate (Liqi and Theresa, 2002; Wittgen et al., 2012).

BCRP is expressed in the liver, small intestine, mammary gland, colon, pancreas, bladder, brain, placenta, kidney, and prostate (Maliepaard et al., 2001; Fetsch et al., 2006; Dankers et al., 2012; Anger et al., 2012). The expression of BCRP in vital strategic tissue signifies its role in distribution, absorption, and elimination of the anticancer drugs that are BCRP substrates. BCRP has been an important multidrug resistance protein due to the cross-resistance it offers to diverse classes of chemotherapeutic agents (Natarajan et al., 2012). Chemotherapeutic agents such as methotrexate, erlotinib, mitoxantrone, danusertib, lapatinib, doxorubicin, and topoisomerase I inhibitors such as topotecan are the substrates of BCRP and gefitinib (Allen et al., 1999; Vlaming et al., 2011; Marchetti et al., 2008; Balabanov et al., 2011; Polli et al., 2008; Chen et al., 2011). Preclinical studies conducted in knockout mice have revealed that BCRP can reduce the maximum plasma concentration of its substrate by a factor of three (Seamon et al., 2006).

1.3.2 Presystemic metabolism

Oral bioavailability is defined as the cumulative function of the amount of the drug absorbed from the gastrointestinal tract and the enterohepatic circulation and the extent of the drug available following first-pass hepatic metabolism. Similarly, hepatic and gastrointestinal availability of the drug can be defined as the fraction of the drug that escapes the metabolizing effects of the liver and gastrointestinal tract (Wu et al., 1995; Thanki et al., 2013). The intestine and liver contain enzymes that metabolize a significant amount of the drugs before they reach the systemic circulation. Because such enzymes are present in both glands, it is difficult to differentiate the relative importance of hepatic and intestinal enzyme–mediated metabolism of the drugs (van Herwaarden et al., 2009). Anticancer drugs taken orally face the issue of presystemic metabolism, preventing a therapeutic dose from being achieved and reducing the drugs’ clinical outcomes.

Cytoplasm on the endoplasmic reticulum contains extrahepatic microsomal enzymes that contribute to intracellular metabolism in the gut. The cytochrome P450 3A family, especially CYP 3A4, phase I metabolizing enzymes, is present in the enterocytes that metabolize drug substrates in the gastrointestinal wall. Phase II metabolizing enzymes such as esterases and glutathione-S-transferases have also been reported to be present in the intestine (Thummel et al., 1997). Although the intestinal epithelium is the primary site for the preabsorption of metabolism, it contributes to lower bioavailability of ester-type drugs such as capecitabine and therapeutic peptides. It has also been reported as a key target for delivery of amide or ester prodrugs (Tabata et al., 2004).

When the drugs have been absorbed from the gastrointestinal tract, they are entered into the enterohepatic portal vein and delivered to liver. Some amount of absorbed drug gets metabolized in the liver during first-pass hepatic metabolism. Because the liver contains different types of enzymes, it is known as a metabolic clearinghouse both for endogenous substances such as steroid hormones, cholesterol, proteins, and fatty acids and for xenobiotics (Brunton et al., 2006). First-pass metabolism of anticancer drugs contributes significantly toward decreasing their oral bioavailability. Tamoxifen is one example of an anticancer drug that faces first-pass metabolism upon its oral delivery (Shin et al., 2006). The situation is further worsened in the case of anticancer drugs that are substrates for both P-gp and cytochromes, leading to significant decrease in oral bioavailability. These both work in close correlation with each other. The expression of P-gp increases in a similar flow from proximal toward the small intestine when the levels of CYP 3A4 decrease. Thus they should be carefully considered when oral systems are designed for the delivery of their substrates (Mouly and Paine, 2003).

The dihydropyrimidine dehydrogenase (DPD) enzyme is found in the liver and intestinal wall. It is also responsible for the presystemic metabolism of various oral chemotherapeutic agents, resulting in the erratic and decreased oral bioavailability of its substrate anticancer drugs. One such drug is fluorouracil, which does not achieve significant plasma concentration after oral delivery. Only 28% oral bioavailability is achieved for the drug with wide variations when given orally, indicating the degradation effects of PDP (Spector et al., 1993; Koolen et al., 2011; Baker et al., 1996; Chirstophidis et al., 1978). Moreover, polymorphisms in genes responsible for DPD activity are responsible for interpatient variability. The intrapatient variability is associated with the circadian rhythm of DPD activity throughout the day (Schöffski, 2004).

1.3.3 Nonlinear pharmacokinetics of anticancer drugs

The oral bioavailability of anticancer drugs is also reduced by their higher absolute oral doses, as limited area is available for their absorption in the intestine as compared to saturation. The transporter proteins of the intestinal gut get saturated with the drug and their further drug uptake capability decreases, leading to nonlinear pharmacokinetics (Harvey et al., 1986). An increase in the oral dose will not increase exposure if the maximal rate has been achieved for the process. Etoposide is one example of an anticancer drug exhibiting nonlinear oral pharmacokinetics. When given orally at a 100 mg dose, etoposide exhibits a bioavailability of 76% as compared to 48% for an oral dose of 400 mg (Hande et al., 1993; Chabot et al., 1996).

1.3.4 Hepatic impairment and malabsorption

Development of liver dysfunction and gastrointestinal disorders are very common in cancer patients. The mechanisms involved in the development of these disorders can be related to hormonal secretions, surgical resections in the gastrointestinal tract, or adverse side effects associated with chemotherapy (Sharma, 2001). Damage to mucosa of the small intestine and its malabsorption significantly reduce the absorption of orally administered chemotherapeutic agents (Dias et al., 1998; Banna et al., 2010). Alterations in presystemic metabolism in hepatic regions as well as transport systems also change anticancer drugs’ pharmacokinetics after their oral intake (Eklund and Mulcahy, 2005). Cancer patients with impaired hepatic function should be assessed to establish the pharmacokinetics patterns and to ensure safety by analyzing the drug fractions that are metabolized due to impairments and adjusting treatment strategies accordingly (LoRusso et al., 2012; US Food and Drug Administration, 2016).

1.3.5 Genetic variations

Differences in the levels and activities of metabolism enzymes and transporter proteins in the liver and gut cause intrapatient and interpatient variability in the bioavailability of orally administered anticancer drugs. This variability occurs due to variations in the genetic constitutions of patients. The phenomenon of polymorphism in genes encoding for metabolism enzymes and transporter proteins influences the elimination and absorption of the oral drug with a low permeability. This can be better understood from cancer patients with a homozygous C1236T polymorphism in the ABCB1 gene (ABCB1*8) that reduce the docetaxel clearance significantly (Bosch et al., 2006). Coexistence of CYP 3A5*3 and CYP 3A4*1 has been shown to cause increased clearance of docetaxel (Baker et al., 2009). The clinical implications of pharmacogenetic variability in drug transport, phase I and II drug metabolism, and pharmacodynamics are described in a four-part series on pharmacogenetics with greater focus on chances for patient-tailored anticancer therapies (Deenen et al.,