Marine Natural Products with P-Glycoprotein Inhibitor Properties

P-glycoprotein (P-gp) is a protein belonging to the ATP-binding cassette (ABC) transporters superfamily that has clinical relevance due to its role in drug metabolism and multi-drug resistance (MDR) in several human pathogens and diseases. P-gp is a major cause of drug resistance in cancer, parasitic diseases, epilepsy and other disorders. This review article aims to summarize the research findings on the marine natural products with P-glycoprotein inhibitor properties. Natural compounds that modulate P-gp offer great possibilities for semi-synthetic modification to create new drugs and are valuable research tools to understand the function of complex ABC transporters.

. Flippase mechanism. P-gp interchanges molecules from the inner leaflet of the membrane to the outer leaflet, in order to maintain a concentration balance on both sides.
Conversely, the -hydrophobic vacuum cleaner‖ model (currently the most accepted) suggests that P-gp removes hydrophobic molecules that are within in the membrane by a -vacuum cleaner‖ device ( Figure 2) [12][13][14]. The high-resolution crystal structure of a mouse P-gp does not differentiate between the models, but rather, could be used to support either of them. The protein structure contains two portals within the P-gp cavity that connect to the inner leaflet of the membrane and would allow hydrophobic drugs to pass directly from the lipid bilayer to the cavity [14]. (1) Substrates pass through the membrane to the lipid bilayer; (2) substrates can enter the P-gp through -portals‖ that pass the substrate from the lipid bilayer in to the P-gp internal cavity; (3) substrates bind to the drug-binding site (DBS); (4) P-gp transports the substrate to the outside of the cell.

Role of ATP in Protein Activation
In the absence of exogenous substrates, P-gp has an intrinsic ATPase activity [15]. Moreover, P-gp substrates can positively or negatively modulate this activity and, consequently, the rate of hydrolysis of ATP [16]. Compounds that modulate the P-gp ATPase activity have been categorized into three groups: (1) Compounds that stimulate basal ATPase activity at low concentrations and inhibit the activity at high concentrations (e.g., vinblastine, verapamil and paclitaxel); (2) compounds that increase ATPase activity in a dose-dependent manner (e.g., bisantrene, valinomycin and tetraphenylphosphonium); and (3) compounds that inhibit both basal and compound-stimulated ATPase activity (e.g., cyclosporin A, rapamycin and gramicidin D) [16].
A particularly important aspect to consider in the mechanism of P-gp efflux function is the role of ATP in protein activation. Higgins and coworkers proposed that two molecules of ATP are required in the first step of protein activation [17]. Siarheyeva and colleagues suggested a mechanism that also involves two molecules of ATP, but one ATP molecule is strongly bound and the other weakly bound to the protein, affording two specific substrate-binding domains, one of high affinity and one of low affinity [18]. Sauna and coworkers proposed another model in which one molecule of ATP activates the efflux pump of P-gp to move drugs out of the protein, and a second ATP molecule returns P-gp to its original conformation [19].
All these models predict that inhibiting the protein ATPase activity can disrupt P-gp efflux function. However, this is not a general rule, because some P-gp inhibitors increase the protein ATPase activity [20][21][22][23]. Furthermore, initial investigations have suggested that P-gp inhibitors should bind to the protein nucleotide binding site (NBS) to inhibit ATPase activity, but one study of a flavonoid-type inhibitor found that the compound stimulated the activity of ATPase without binding to the protein NBS [22].
In general, P-gp inhibitors work in one of the three ways: 1. Promoting a conformational change in P-gp that blocks the ATP binding site and, subsequently, ATPase function; 2. Promoting a conformational change in P-gp that enhances ATP binding, but concurrently blocks the substrate binding site; 3. Inactivating the substrate binding site without inducing any conformational changes, e.g., stereo-isomers of cyclic hexapeptide inhibitors QZ59-RRR and QZ59-SSS [24].
P-gp was first discovered in 1976 for its role in MDR in cancer; it is overexpressed in several human tumors and is an important barrier to success in cancer treatments [3][4][5][6][7][8]10]. Interestingly, P-gp in tumors also appears to provide resistance to apoptosis induced by different stimuli, including serum starvation, Fas, TNF and UVB-and γ-irradiation [5,8]. The mechanism whereby P-gp inhibits apoptosis is still unclear [5,8].

P-gp Inhibitors
Over the years, identifying small molecules that interfere with the activity of P-gp has taken relevance, because blocking its pump function could reduce the effective concentration of drugs administered in the treatment of cancer, HIV, parasitic diseases and other diseases. On the basis of their specificity, affinity and toxicity, P-gp inhibitors are categorized into three generations ( Table 1). The first generation of inhibitors are metabolites that already have a clinical use, e.g., verapamil (calcium channel blocker drug) and cyclosporine A (immunosuppressant drug), and, subsequently, were tested against P-gp and found to inhibit the enzyme. These drugs required high concentrations to inhibit P-gp, and for this reason, they were not approved as inhibitor P-gp drugs [31,32].
Second generation inhibitors are compounds without previous therapeutic use and that have a higher affinity for P-gp than the first generation compounds. The problem with these metabolites is that they are quickly metabolized by the CYPA4 enzyme, thus altering their pharmacokinetics and reducing their efficacy. It is important to point out that these inhibitors are designed to have lower toxicity than the compounds belonging to the first generation, despite retaining certain undesirable toxicity features that limit their pharmacological use [33][34][35]. The third generation inhibitors are compounds that were obtained using combinatorial chemistry and subsequent structure-activity relationship studies to identify compounds that inhibit P-gp with high specificity and low toxicity. These P-gp inhibitors have a potency of about 10-fold more than the earlier generations of inhibitors. These compounds are not inhibited by the CYPA4 enzyme, and therefore, they do not exhibit altered pharmacokinetics [36,37].
Inhibitors belonging to one of the three generations exert their effect by one of the following mechanisms ( Table 2): (1) Disrupting the hydrolysis of ATP [38][39][40][41][42][43][44][45][46][47]; (2) altering P-gp expression [48][49][50][51][52][53][54][55]; and (3) reversible inhibition or competition for a binding site, as demonstrated by photoaffinity labelling [56][57][58][59][60][61][62][63][64][65]. One of the most common mechanisms displayed by classical P-gp inhibitors is competition for drug binding sites. However, P-gp has multiple binding sites, making it difficult to design target-based inhibitors. The poor selectivity shown by inhibitors with this mechanism could be due to P-gp having multiple binding sites. It is also difficult to detect general functional groups that modulate inhibitory activity against P-gp; however, it has been possible to obtain active functional groups from specific pharmacophores. One of the most interesting studies focused on identifying functional groups of active molecules, which was performed with 27 digoxin transport inhibitors in Caco-2 cells, found that two hydrophobic groups along with a hydrogen-bond acceptor group and an aromatic core were required for P-gp inhibition [66] This finding is relevant, because many previously reported P-gp inhibitors also possesses the majority of these features. It is also important to highlight that peptides similar to cyclosporine A (in size and amino acid composition) could have the inhibitory properties of P-gp, as in the case of valspodar and kendarimide A.
Many inhibitors of P-gp, especially within the third generation compounds, have been tested in clinical trials to assess their pharmacological potential (Table 3). Unfortunately, most of them have failed, because they displayed non-specific toxicity [8]. Among the negative factors that prevented success are: (1) High variability in the response rate associated with P-gp inhibitors, which is related to levels of P-gp expression and the co-expression of other ABC transporters [1,8,67]; (2) the pharmacokinetic interaction between the P-gp inhibitor and the other co-administered drugs, which lead to an increase in drug toxicity [8]; (3) the increase in plasma concentrations of a co-administered drug by interfering with its metabolism or excretion [8]; and (4) the increase in the toxicity of a co-administered drug in healthy tissues by inhibiting the basal activity of P-gp [8]. Therefore, there is an urgent need for identifying new, more effective and non-toxic P-gp inhibitors.

Inhibitors from Marine Sources
Oceans cover around seventy percent of the Earth's surface and represent a resource of huge dimensions for natural product chemistry. This media contains nearly eighty percent of the biological diversity of life on the planet. Studies over the past 30 years have led to the discovery of thousands of new compounds from marine sources, which have shown a wide range of biological activities [68][69][70].
Several marine compounds or analogs inspired by marine natural products have been approved for clinical use, including vidarabine (for the treatment of a recurrent epithelial keratitis caused by herpes simplex virus type 1 and 2 and superficial keratitis), cytarabine (for cancer), ziconotide (for the treatment of severe chronic pain in patients with cancer or AIDS), trabectedin (for use as an anticancer agent against soft tissue sarcoma) and halaven (for metastatic breast cancer) [68][69][70].
In this article, we systematically review several marine natural products with P-gp inhibitor properties ( Table 4). As mentioned, this protein is a major cause of drug resistance in cancer, some parasitic diseases, epilepsy and other disorders. The modulation of targets of P-gp by natural or synthetic compounds offers great possibilities for the discovery of new drugs and valuable research tools to understand the complex ABC transporters.

Inhibitors from Tunicates
The compound, ecteinascidin 743 (ET-743, 1) (Figure 3), isolated from the Caribbean tunicate, Ecteinascidia turbinata [71,72], showed good anti-cancer in vitro activity against mouse lymphocytic leukemia (L1210) cells with a half maximal inhibitory concentration (IC 50 ) values of 0.5 ng/mL. ET-743 partially reverses resistance to doxorubicin and vincristine in MDR epidermal carcinoma (KB-C2 and KB-8-5) P-gp/multidrug resistance 1 (MDR1) overexpressing cancer cell lines. A greater intracellular accumulation of doxorubicin and vincristine (up to 122 and 22 fold, respectively) were observed in both cells when pretreated with non-toxic concentrations of 1. However, photoaffinity labeling experiments showed that overcoming doxorubicin/vincristine resistance was not a result of the direct inhibition of P-gp activity [73]. Because of these beneficial effects in cancer treatments, 1 has received orphan drug designation specifically for soft tissue sarcoma treatment in the United States and ovarian cancer treatment in the United States and Europe [74]. Lamellarins are a group of polyaromatic alkaloids originally isolated from Lamellaria sp. [75] and later from the ascidian, D. chartaceum [76,77], the sponge, Dendrilla cactos [78,79], and some species of unidentified ascidians [80][81][82]. This class of compounds has shown diverse biological activities, including cytotoxicity [75,83,84], immunomodulating activity [77], inhibition of HIV integrase [83] and, critically, the ability to render some MDR cancer cell lines susceptible to anti-cancer treatments [84].
Lamellarin I (2) (Figure 3) presented a better chemo-sensitizing activity than verapamil (nine to 16 fold higher) in doxorubicin-resistant human colon adenocarcinoma (Lo Vo/Dx) cell line. In addition, 2 increases the cytotoxicity of doxorubin, vinblastine and daunorubicin in a concentration-dependent manner in MDR cells. Compound 2 exerts this effect through a direct inhibition of the P-gp pump function, as demonstrated by the accumulation of Rhodamine 123 in Lo Vo/Dx cells [84].
The patellamides are thiazole-and oxazoline-containing cyclic octapeptides isolated from Lissoclinum patella that show several biological activities, including cytotoxicity and reversing resistance in the MDR human leukemic (CEM/VLB100) cell line against vinblastine, colchicine and adriamycin [85,86]. The cytotoxicity of patellamide-type compounds may be because of conformational restrictions set by the presence of the heterocycles and their ability to intercalate DNA [86]. Of this family of compounds, patellamide D (3) (Figure 3) showed the best activity in reversing MDR; it enhanced by 66, 2.8 and 1.4 fold the activity of vinblastine, adriamycin and colchicine, respectively. The activity of 3 is similar to verapamil, a well-known P-gp inhibitor [87].

Inhibitors from Sponge
A novel polyhydroxylated sterol acetate, agosterol A (4) (Figure 4), was isolated from the marine sponge, Spongia sp. [88]. This compound completely reversed MDR in human KB carcinoma cells overexpressing an MRP1 (a membrane glycoprotein) [88,89]. In order to obtain the mechanism of action of 4, accumulation and efflux experiments were performed using KB-C2 and human carcinoma overexpressing MRP1 (KB-CV60) cell lines [89].  (2,4-dinitrophenyl)glutathione in inside-out membrane vesicles from KB-CV60 cells [90]. Taken together, the data indicate that 4 inhibits the drug efflux modulated by P-gp and MRP1.  Discodermolide (5) (Figure 4) is a polyketide that was discovered in the marine sponge, Discodermia dissoluta, due to its immunosuppressive and anti-tumor activities [91][92][93]. Interestingly, 5 has the same mechanism of action as taxol (it blocks the cell cycle at the G"/M checkpoint and induces apoptosis), but is more potent against several types of cancer cell [94]. Additionally, 5 is active against taxol-resistant cells. Normally, MDR ovarian carcinoma (A2780AD) and paclitaxel-resistant colon carcinoma (SW620AD-300) cell lines are 25-89 fold more drug-resistant than their parenteral lines. Compound 5 drastically decreased multidrug resistance to taxol in both cell lines, with IC 50 values in the range of 70 nM for SW620AD-300 (compared to 260 nM for taxol) and 580 nM for A2780AD cells (compared to 3900 nM for taxol) [94].
A mixture of three structurally-related esters (irciniasulfonic acids, 6-11) (Figure 4) were isolated from the marine sponge Ircinia sp. The irciniasulfonic acids mixture are found to reverse MDR at 33 µg/mL against P-gp overexpressing derived from human cancer KB cells (KB/VJ300) cells in the presence of 10 ng/mL of vincristine. It was subsequently discovered that a simple chain of deacylirciniasulfonic acid (9) (Figure 4) was 13 fold more potent than the other irciniasulfonic acids (IC 50 values of 3 μM and 38 μM, respectively) [95,96].
Kendarimide A (12) (Figure 4), isolated from the marine sponge, Haliclona sp., reversed multidrug resistance in a human carcinoma cell line overexpressing P-gp [97]. Compound 12 completely reversed the resistance to colchicine in KB-C2 cells at a 6 μM concentration. A mixture of 6 µM of 12 and 0.1 µg/mL of colchicine inhibited the growth of KB-C2 cells by 87%, while 12 showed no inhibitory activity against human epidermoid carcinoma (KB-3-1) cells at a 6 µM concentration. It is important to point out that cyclosporin A, a potent peptide with activity against multi-drug resistant P-gp, is composed of a similar number of amino acid residues as 12. This might indicate that peptides having an analogous amino acid length have the ability to reverse multidrug resistance [97].
Sipholenol A (13) (Figure 4) is a triterpenoid isolated from the Red Sea sponge, Callyspongia siphonella [98]. This sponge is a prolific producer of about 30 different triterpenes grouped into four major families: Sipholane, siphonellane, neviotane and dahabane. The sipholane family has the ability to reverse P-gp-mediated MDR in some cancer cells [98,99]. Compound 13 enhanced the cytotoxicity of three well-known P-gp substrates (colchicines, vinblastine and paclitaxel) in KB-C2 and MDR human cervix carcinoma subclone derived from KB-3-1 (KB-V1) cells. In this experiment, 13 decreased the multidrug resistance of these cells in a concentration-dependent manner [100]. Finally, it was found that 13 increases the accumulation of paclitaxel by inhibiting the P-gp efflux function, stimulates ATPase activity, inhibits the photolabeling of P-gp using [125I]-iodoarylazidoprazosin as the transport substrate and does not affect P-gp expression. Further research demonstrated that sipholenone E (14), sipholenol L (15) and siphonellinol D (16) (Figure 4) showed similar activity to 13 [101].

Inhibitors from Cyanobacteria and Alga
Welwitindolinones are a group of alkaloids isolated from the cyanobacteria, Hapalosiphon welwitschii. N-Methylwelwitindolinone C isothiocyanate (17) (Figure 5) enhanced the cytotoxicity of two anticancer drugs, actinomycin D and daunomycin, in vinblastine-resistant ovarian carcinoma (SK-VLB-1) cell lines. Additionally, 17 increased the activity of anticancer drugs vinblastine, taxol, actinomycin D, colchicine and daunomycin in MDR ovarian adenocarcinoma (NCI/ADR-RES) cell lines, formerly known as MDR breast carcinoma (MCF-7/ADR) cells. Another member of this alkaloid family, welwitindolinone C isothiocyanate (18) (Figure 5), presented a weak activity, indicating that the methyl group is important for biological activity. Moreover, when an isonitrile group replaced the isothiocyanate group, the compound was inactive, demonstrating that the isothiocyanate group is also important for the biological activity shown by these alkaloids [102]. Two unusual brominated diterpenes, parguerenes I (19) and II (20) (Figure 5), obtained from the Australian marine red alga, Laurencia filiformis, are non-cytotoxic inhibitors of P-gp and MRP1 mediated drug efflux. These two small molecules exhibited a dose-dependent reversal of P-gp mediated efflux of vinblastine, doxorubicin and paclitaxel MDR, without altering the levels expression of P-gp. This suggests that 19 and 20 inhibit P-gp efflux by a novel mechanism of action or at least by a mechanism this is very different to other known inhibitors, such as verapamil and cyclosporin A, due to 19 and 20 interacting with and upsetting the extracellular antibody binding epitope of P-gp. A detailed analysis of the structure-activity relationship between 19 and 20 indicates that it is possible to manipulate and optimize the core pharmacophore of these molecules in order to increase their activity [103].

Inhibitors from Bryozoans
From a species of bryozoan, Bugula neritina, have been isolated a group of macrolide lactones called bryostatins. These compounds are potent modulators of protein kinase C, and they are anticancer and memory enhancing agents [104,105]. Probably the most important member of this compounds family is bryostatin 1 (21) (Figure 6), which is a modulator of protein kinase C with a potency similar to that of the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA) [106,107]. Compound 21 also has the ability to modulate the P-gp mediated MDR. In an experiment using two cell lines overexpressing a mutant MDR1 encoded P-gp, colchicine-resistant MDR clone of KB cells (KB-C1) and human epitheloid cervix carcinoma (HeLa) cells transfected with an MDR1-V185 build (HeLa-MDR1-V185) that contains a valine instead of a glycine in position 185, it is found that 21 reversed the resistance developed against vinblastine and colchicine by both cells [108]. Additionally, 21 is capable of reversing P-gp mediated MDR in HeLa cells transfected with MDR1-V185 by an increase of the intracellular accumulation of rhodamine 123. From these experiments, it was evident that 21 joins to sites G185 and V185 P-gp, and thus, this compound is able to reverse a mutant P-gp specifically [108]. Figure 6. Inhibitors of P-gp that have been isolated from bryozoans, corals and marine bacteria.

Inhibitors from Marine Bacteria
The bacterium, Nocardiopsis sp. CMB-M0232, was isolated from a sediment sample collected in Australia. A non-saline liquid culture of this bacterium yielded two prenylated uncommon diketopiperazines (DKP): Nocardioazine A (28) ( Figure 6) and nocardioazine B. Compound 28 reversed the resistance exhibited by SW620AD-300 cells equipotent to verapamil. Conversely, nocardioazine B showed no inhibitory activity against P-gp, which could mean that the strange-bridged DKP scaffold formed between one of the nitrogens, and the isoprene external unit might be involved in biological activity [111].

Conclusions
P-gp can expel a broad range of structurally different exogenous compounds out of the cells. For this reason, a very active P-gp transporter could potentially diminish drug delivery to the target organ and has been correlated to treatment resistance, despite peripheral drug concentrations that are within their therapeutic range.
Inhibition of P-gp leads to an increase in the permeability of some target organs. This result could permit administering lower drugs oral doses, and it may help to decrease drug toxicity. As a result, P-gp mediated drug efflux is recognized as a desirable target for therapeutic intervention in order to target and optimize the drug delivery of drugs to tumor cells and physiologically/anatomically isolated tissue compartments.
Marine organisms have proven to be an important source in the discovery and development of compounds that inhibit P-gp. Most of these compounds have demonstrated a reversal of multi-drug resistance in certain cancer cells. Importantly, these compounds have a potential application to be administered in conjunction with drug therapies that have been stymied by overexpressed P-gp, including anticancer agents, β-adrenoreceptor blockers, calcium channel blockers, cardiac glycosides, immunosuppressants, steroid hormones and antiparasitic, among others.
Importantly, the majority of the marine inhibitors outlined in the review had been scarcely studied, and very little is known about their mechanism of action. Extensive research, including that of a rational drug design to create derivatives with higher activity, less toxicity and less pharmacokinetic interactions, would lead to more clinically viable drug candidates. Nevertheless, all these agents represent new research tools for the discovery and development of efficient P-gp inhibitors, which may have potential use on their own or in combination with other therapeutic agents for the treatment of various diseases of clinical relevance.