**1. Introduction**

Reduce in the remedial rate of most of the diseases and the decline in the therapeutic efficacy of most of the anti-neoplastic and anticancer drugs is due to the phenomena called drug resistance [1]. The main ambiguity to be found out to increase the efficacy of the drugs is to overcome this phenomenon. The drug resistance is not observed to a single chemotherapeutic drug but to a broad range of structurally and functionally different drugs. The exposure to a single therapy for a long time and the recurring use of the medication leads to drug resistance will leads to decrease in the therapeutic efficacy of the drugs. Even if the dose is altered there would not produce any momentous changes in this phenomena [2]. When the human acquired resistance to the drugs it is termed as drug tolerance. This was usually seen of two type's pharmacokinetic drug tolerance and pharmacodynamic drug tolerance.

#### **2. Metabolic or pharmacokinetic tolerance**

Followed by the entry of the drug in our body with the time it gets absorbed into the bloodstream, which is then carried out and distributed to various other sites, additionally gets disintegrated into various segments and eventually gets excreted from the body. All these factors determine the potency, side effects and duration of action of the drugs. The major reason for the pharmacokinetic tolerance is when the drug fails to maintain its minimum therapeutic concentration at the target site. Where, in this case the enzymatic action of cytochrome P450 (CYP450) produces the effect. This type of tolerance is mainly determined with the oral dosage form which produces first pass metabolism. Induction of the enzymes are the major reason behind the drug resistance which is further accompanied with various other factors which is further disused in this chapter in detail.

#### **3. Pharmacodynamic tolerance**

When the cellular feedback to a substrate is concentrated the development of the pharmacodynamic tolerance eventually occurs. The principle reason behind pharmacodynamic tolerance is when the therapeutic concentration of the substrate to the binding receptors reaches above the maximum therapeutic concentration which eventually results in the desensitisation of the receptors [3]. Other possibilities include the decline in the receptor density which is mainly associated with the receptor agonist and the modification in the action of the possible firing rate. Generally the drug resistance will occur subsequent to the incessant exposure to the drug, but instant tolerances were also observed in rare cases [4].

#### **4. Factors responsible for drug resistance**

There are a variety of drug resistance usually seen such as anticancer resistance, anti-human immunodeficiency virus (HIV) drug resistance, antibiotic resistance, anti-tubercular drug resistance, anti-malarial drug resistance and anti-microbial drug resistance. The most part of the drug resistance is caused by MDR proteins, where in this P-gp (P-glycoprotein) involves in producing a major aspect in reducing the drug efficacy in most of the treatment. P-gp, which is a trans-membrane (TM) glycoprotein physiologically, expressed in the parts of the body such as kidney, liver, pancreas, colon and jejunum [5], it also expresses its role eventually in the brain capillary epithelial cells. The core function of P-gp is to safeguard the cells and restrict the entry of xenobiotics and toxic substances. But its action is overexpressed in the diseased cells by restricting the entry of the drugs hence its action should be inhibited in such cases for the success in the therapy.

#### **5. Transporters and its family**

P-gp is an efflux protein system associated with the ATP binding cassette (ABC) sub-family B membrane or Multi-drug resistance 1 (MDR1) or cluster of differentiation 243 which belong to ABCB (MDR) super family of ABC transporter [6]. ABC gene indicates the leading family of TM protein, originated mainly in the intercellular membrane or the plasma membrane. By using the energy from the ATP the transport mechanism across the cell membrane is initiated. In humans

#### *P-Glycoprotein Efflux Transporters and Its Resistance Its Inhibitors and Therapeutic Aspects DOI: http://dx.doi.org/10.5772/intechopen.90430*

around 49 ABC transporters are observed [7], where MDR1A, MDR1B and MDR2 [8] are usually identified in animals and MDR1 and MDR3 [9] which belong to the P-gp gene subfamily is mostly seen in humans. Where MDR1 (P-gp) is widely seen all over the body and efflux a wide range of drugs over the plasma membrane and MDR3 (P-gp) is predominantly observed in the liver, canalicular membrane of the hepatocytes and is accountable for the phosphatidylcholine secretion into the bile [10]. Even though the action of MDR3 or P-gp in the efflux transport mechanism is observed, their direct action in drug resistance is restricted.

The efflux action exhibited by P-gp is having a greater importance since it can protect our body from the entry of the toxins and xenobiotics into the cells mainly to brain, placenta and gonads and eliminating the waste products through the urine [11], by facilitating the energy driven with the ATP hydrolysis the amphipathic drugs are mainly eliminated by the efflux action of the P-gp.

P-gp role is mainly observed in the body parts such as kidney, intestine, liver [12], testis and brain [13]. The entry of the xenobiotics into the blood capillaries are mainly minimised by the localised action of P-gp in the luminal membrane of the epithelial cells [14]. Over-expression of the P-gp is the foremost reason behind the failure of the chemotherapy and other treatment strategies where nowadays the researchers are mainly focussed to overcome with this issue. The studies related to the action exhibited by the P-gp are performed in mice, rats [15] and in humans [16]. The substrates which merge with P-gp have unrelated frameworks. The compounds transported by P-gp are considered as substrates, while the compounds that prohibit the role of P-gp are considered as inhibitors.

P-gp was identified first in 1976 in Chinese hamster ovary cells, where it was obtained to demonstrate anticancer resistance [17]. The research data proved that the P-gp have the ability to acquire resistance to the cytotoxic drugs. They also proved that verapamil was helpful to measure the function of the P-gp by using positron emission tomography [18]. P-gp is used for differentiating the transitional B-cells from the native B-cells. Rhodamine 123 and Mito Tracker Dyes are also used for this purpose [19].

### **6. Cellular localisation**

The expression of P-gp is recurrently found predominant in the cancer cells, causing MDR by efflux of lypophilic drug from the cell. P-gp is observed to be overexpressed in renal, colon and adrenal carcinoma, not often in germ cell tumour and lungs even certain in gastric carcinoma, undetectable in breast and endometrial carcinomas. In the normal cells the concentration of the P-gp and its expression is found to be low, but certain cell types like colon, kidney, liver, jejunum and pancreas shows a higher expression of P-gp. While in liver, the broad distribution of P-gp is found on the apical surface of epithelial cells and biliary canalicular front in the small biliary ductules [20]. When it appears in case of pancreas, P-gp exclusively found of the apical surface of epithelial cells of the small ductules. In Kidney the presence of the P-gp is exclusively found on the apical surface of the epithelial cells of the proximal tubules [5]. P-gp expression shows an identical expression in the apical surface of superficial columnar epithelial cells of both jejunum and colon. Where, the expression of P-gp is mostly seen in the surface of cortex and medulla cells in the adrenal glands. Its presence is found out with both secretary and excretory action in the specialised epithelial cells, placental trophoblast [21] and endothelial cells of capillary cells at blood-tissue barrier sites. Its presence is also found in the epithelial cells of bronchi and gastrointestinal tract, prostate gland, salivary glands and sebaceous gland of the skin.

The research reports proved the occurrence of P-gp in the human fetus is with the significance in the regular performance of various organs in the initial stage of embryo development [22]. The evidential reports from the research work conducted in the tissues of rodent and humans [23] came to a conclusion that the expression of the P-gp in the normal tissues are much lower than compared to that at the epithelial cell lining of small Intestine, colon, pancreatic duct, proximal tubules of kidney, bile ducts and adrenal gland [24]. P-gp over-expression is also observed in the secretary epithelial cells in the endothelium of the pregnant woman and also in the placenta for protecting the fetus [25].

### **7. Kinetics of P-gp**

#### **7.1 Absorption**

The major reason behind the multidrug resistance in the cancer cells is the action exhibited by P-gp. P-gp transports a wide range of structurally and functionally different cytotoxic compound out of the cell by using the energy driven from ATP. Altering in this property caused by P-gp is a vital approach for overcoming the MDR part and to increase the therapeutic efficacy during the treatment. The incorporation of the P-gp inhibitors with the resistant drugs can be helpful towards suppress the expression of the P-gp. This situation of resistance part not only happens with the chemotherapeutic drugs but also for the treatment strategy used for other conditions also [26]. The over-expression of P-gp in the diseased cells as compared to that of the normal cells is the major reason behind this phenomenon. Research work carried out came with a conclusion that P-gp is the major cause for the antibiotics resistance part also. P-gp trims the overall permeability of the drug to reach the target site by reducing the minimal therapeutic concentration level [27].

#### **7.2 Distribution**

P-gp acts as an integral element in the distribution of the drugs. Its action is noticed in the blood brain barrier and also the placental barrier. It can accomplish its effect on the distribution of various therapeutic agents also reduce its activation in the body. The role of P-gp in the brain turned into a point where it resists the entry of the neuro-toxic drugs into the brain and hence sustains the penetration of central nervous system (CNS) agent delivery in the brain [28]. A modulator is in need to inhibit the action of P-gp for improving the efficacy of CNS drugs to the target site including for the Parkinson's and Alzheimer's disease. When it comes towards P-gp inhibition the P-gp also protect the fetus from the entry of the foreign bodies and toxic substances through the placenta [29, 30]. Hence by considering these aspects the inhibition of the P-gp is to be made in a careful manner by not altering the protective mechanism in the normal cells [31].

#### **7.3 Metabolism**

The enzymatic activity of CYP3A4 and the efflux mechanism by P-gp together play the role for the decreased therapeutic efficacy and bioavailability of the drugs administered through the oral route. These are major defence system in intestine which acts as a protective barrier from the entry of the toxic substances and the xenobiotics. These proteins are mainly over-expressed in enterocytes and hepatocytes and also establish its role in first pass metabolism of the drugs.

*P-Glycoprotein Efflux Transporters and Its Resistance Its Inhibitors and Therapeutic Aspects DOI: http://dx.doi.org/10.5772/intechopen.90430*

For supporting this statement piperine can be taken as an example. The flavonoid piperine is obtained from black pepper. It have the ability to act as a natural inhibitor of P-gp and CYP34A when and can increase the therapeutic efficacy of the drugs when co-administered [32]. The dose of the piperine administered should be ideal and reaches the minimum therapeutic concentration to produce this effect. Another example is the action exhibited by grape fruit juice. When grape fruit juice and saquinavir is co-administered, the therapeutic concentration of the parent drug can be increased when administered during oral route.

#### **8. Elimination**

#### **8.1 Renal excretion**

The process of glomerular filtration, tubular secretion and reabsorption involve in the mechanism of renal excretion. P-gp involved in the efflux of the xenobiotics and the waste materials from our body via renal excretion, results in reduced level of the therapeutic drug in the blood plasma. This can also be altered by incorporating the flavonoids with the therapeutic agents. When digoxin a flavonoid and Cyclosporine A is co-administered, Cyclosporine will increase the concentration of digoxin in the plasma by decreasing tubular secretion and glomerular secreation rate [33, 34]. Comparable results are obtained when itraconazole and cimetidine is coadministered [35].

#### **8.2 Biliary excreation**

The excretion of the drugs by the influence of the P-gp can also be altered by using the natural P-gp inhibitors. Quercetin, a natural inhibitor of P-gp can add on to the therapeutic concentration of a wide range of drugs in the target site when coadministered. In turn the therapeutic agents such as Azithromycin, erythromycin, cyclosporine A and doxorubicin have an inhibitory action on biliary excretion of drugs mediated by P-gp [36].

#### **8.3 Antimicrobial drug resistant mechanism**

The mechanism by which the antimicrobial drugs acquiring resistance to the microorganisms are as follows:


#### **8.4 P-gp role in drug resistance**

P-gp has a leading aspect in reducing the bioavailability and distribution of the drugs, where P-gp is over-expressed in intestinal region which act as a substrate to P-gp and reduces its absorption pathway. Hence the therapeutic level of the drugs and the bioavailability of the drugs are not accomplished. On the other hand if the P-gp expression is abscessed then the concentration of the drug in the plasma will reach to supra-therapeutic concentration leading to toxicity related issues [9].

The substrate infiltrate into the P-gp all the way through the protein cytoplasmic side or through the inner leaflet of the membrane. ATP attitudes to the cytoplasmic side of P-gp, pursue with its binding, ATP hydrolysis activates which modify the substrate to be efflux form the cell. The substrate excretion occurs followed with the release of the phosphate from the ATP molecule. A novel molecule of ATP attach to the secondary ATP binding site when adenosine diphosphate (ADP) is discharged. This process will proceed with the hydrolysis and discharge of ADP and a phosphate molecule reset the protein.

#### **8.5 Substrate for P-gp**

P-gp transports a broad range of substrates which are structurally and functionally different from each other. P-gp substrate mainly appear to be lypophilic and amphipathic in nature [37, 38]. For altering the functions of P-gp its inhibitors are mostly generated. The mechanism of action is either by competition with the drug binding sites without hindering the action of ATP hydrolysis of by blocking the ATP hydrolysis process [39]. Recently allosteric mechanism for P-gp mediated transport also added with the other two mechanisms [40]. P-gp substrates are attached with the protein molecule before getting attached or moved to the extracellular membrane leaflets.

#### **8.6 P-gp inhibitors**

The reports from the research work proven that the P-gp has the ability to interact with more than 20 substrates or the modulators. Some of the substrates which are easily transported by P-gp include anthracycline, vinca alkaloids and fluorescent lipids. This binding action of the modulators such as cyclosporine and verapamil are employed for altering the P-gp activity for the chemotherapy. The high flexible and the low specific nature of the P-gp binding pockets could be employed for overcoming the MDR related issues during the chemotherapy [41]. The Classification of the P-gp inhibitors is mainly based on its specificity, affinity and the toxicity. The classification and the division of the inhibitors are mentioned in **Table 1**.

#### **8.7 First generation inhibitors**

The inhibitors belonging to this generation are pharmacologically active in nature and are used in specific treatment. Some of them are reserpine, verapamil, cyclosporine A, yohimbine, quinidine, toremifene and tomoxifene. When we are taking the example of leukaemia cells, the resistance could be inverted by using verapamil [42] for producing an effective action high dose of the drug is given to the patient, results in cardiovascular toxicity [43]. Hence these inhibitors are replaced with the second generation inhibitors because of their less therapeutic efficacy.

*P-Glycoprotein Efflux Transporters and Its Resistance Its Inhibitors and Therapeutic Aspects DOI: http://dx.doi.org/10.5772/intechopen.90430*


#### **Table 1.**

*Generation of inhibitors with examples.*

#### **8.8 Second generation inhibitor**

The substrates coming in this generation inhibitors are pharmacologically inactive in nature but produce its action on P-gp. This generation inhibitors are developed by structurally modifying the first generation inhibitors for obtaining high specificity, low toxicity and potency. Examples for this generation inhibitors include doxverapamil [44], valspodar (PSC 833) [45], biricodar citrate (VX710) [46], dofequidarfumerate and dexniguldipine. Non immunosuppressant analogues of dox verapamil and cyclosporine A are mainly included in this category. PSC 833 which is the most frequently used inhibitor exhibit 5–10 times more potency as compared to that of cyclosporine A [47]. On the other these inhibitors have greater affinity and inhibitory activity towards the ABC transporters and CYPA4 enzymes.

#### **8.9 Third generation inhibitor**

To overcome the problems associated with the first and second generation inhibitors, the third generation inhibitors are generally developed. The main advantage of using the third generation inhibitors are their less toxic effect as compared to that of the first two generation inhibitors and their specificity and effectiveness towards the P-gp. They are found with no pharmacological interaction. They do not possess any kind of pharmacological interaction with the chemotherapeutic agents and found to be 200 times more potent than first two generation inhibitors. Examples include Zosuquidar (LY335979), [48] Tariquidar (XR9576), Laniquidar (R101933), [49] Elacridar (F12091), ONT-093, [50] Mitotane (NSC- 38721), [51] annamycin, [52] HM30181,R10933, [53] HM30181, Biricodar. From the 3D QSAR and QSAR activities it is reported that the structure of the inhibitors are mainly responsible to produce the inhibitory activities. Studies reported that the heterocyclic ring of the tariquidar near to the antranilamide ring is responsible to produce the inhibitory activity. But the recent studies reported that tariquidar is having both substrate as well as inhibitory activity on P-gp [54, 55].

#### **8.10 Fourth generation- natural inhibitors**

Owing the toxicity issues and the restricted therapeutic caused with the synthetic inhibitors, the natural inhibitors are mainly developed which includes the dietary supplements also. The natural compounds and the food extracts are revealed with an effect on P-gp to reverse MDR and also exhibit anticancer property.

### **8.11 Spices**

From the ancient period itself the use of the spices are predominant as preservative and colouring agents. The phytochemicals constituent in spices are studied for the cure of various ailments and for the management and reversal of MDR caused by P-gp [56].

#### **8.12 Curcumin**

Curcumin is used as anti-oxidant, anti-inflammatory agent, anti-infective and anticancer agent [57]. It also exhibits an additional use of reversing the MDR caused by P-gp. It procure this action by acting on P13K/Akt/NF-kB [58] pathway in L1210 MDR leukaemia cells in the mice model. In 2008 Choi et al came with a conclusion that the combination of curcumin and Adriamycin can overcome the MDR effect caused by P-gp by studying with the western blotting results [59]. Mucoadhesive microemulsion loaded with curcumin has the ability for brain targeting through intranasal route [60].

## **8.13 Piperine**

Piperine is the alkaloid constituent present in a larger proportion in black pepper, which is consumed by the population all over the world and added in their diet. Piperine was found with the activity on altering the MDR by inhibiting ABC transporters [61].

### **8.14 Capsaicin**

Capsaicin is found abundant in red chilli, exhibit anticancer and inhibitory activity on P-gp. It potentiates anticancer activity of vinblastine by modulating P-gp. It has the ability to act on β-catanin and NF-KB pathways [62].

### **8.15 [6]-Gingerol**

Ginger is constituted with a major polychemical compound [6]-Gingerol, which add on its spicy taste. It also inhibits β-catanin and NF-KB pathways like the action exhibited by capsaicin, but the actual mechanism of action is still not known [63].

#### **8.16 Carnosic acid**

Carnosic acid is the major phenolic constituent found in the leaves of rosemary. Carcinoic acid can act as a substrate to P-gp by stimulating the ATP activity by competitively binding with the ATP binding site [64].

#### **8.17 Procyanidine**

This compound composes a major constituent in tea leaves and grape seeds. It exhibits both chemopreventive activity and also antiproliferative effect [65]. It can inhibit NF-KB and translocate YB-1 into the nucleus through dephosphorylation of ERK1/2 and AKT [66].

#### **8.18 Limonin**

The citrus fruits are constituted higher with this crystalline compound. It acts as a P-gp inhibitor in leukaemia, melanoma and colon cancer cell lines. It elevates the accumulation of the Rhodamine 123 and doxorubicin inside the cells. When administered in a concentration of 20 μm it has the ability to increase the anticancer activity of the doxorubicin when studied with CED/ADR5000 Caco-2 and leukaemia cell lines [67].
