The objective of this dissertation is to study the mechanism by which HIV protease inhibitors enter into the central nervous system (CNS) and breast milk of rats, and what effects MDR modulators have on the distribution and metabolism of HIV protease inhibitors. The transporter P-glycoprotein (P-gp) has been shown to limit the distribution of HIV protease inhibitors into the CNS of rodents. This thesis examined the effects of GF120918, an MDR modulator, on the CNS distribution of amprenavir, an HIV protease inhibitor, in rats. GF120918 significantly increased the unbound CNS concentrations of amprenavir without altering the unbound blood concentrations of amprenavir. The results of these studies show that GF120918 can inhibit P-gp at the blood brain barrier (BBB) to increase the unbound CNS concentration of amprenavir and potentially other HIV protease inhibitors. Many first generation MDR modulators inhibited both P-gp transport and CYP3A metabolism. Therefore, a principal goal of this thesis was to determine if GF120918 could selectively inhibit P-gp transport without inhibiting CYP3A metabolism. Using in vitro (human) and in vivo (rat) studies, GF120918 selectively inhibited P-gp at the BBB without inhibiting CYP3A metabolism. The transporter MRP1 has been shown to both transport HIV protease inhibitors and expressed in the CNS. Studies contained in the thesis have shown that mrp1 is not localized to the BBB of rats, therefore, mrp1 is unlikely to play a significant role in the distribution of HIV protease inhibitors into the CNS of rats. The distribution of nelfinavir, an HIV protease inhibitor, into rat breast milk was studied in the thesis as a first approach in understanding the extent to which HIV protease inhibitors can accumulate into milk. The concentration of nelfinavir in rat milk was approximately half that of plasma. P-gp protein expression was detected in lactating rat mammary tissue. However, GF120918 showed no effect on the distribution of nelfinavir into rat milk suggesting that P-gp does not play a significant role in the distribution of HIV protease inhibitors into milk.

P-glycoprotein (Pgp), which leads to multidrug resistance in tumour cells, is an ATP-dependent secretory drug efflux pump. In the intestine, as well as at specific other epithelial and endothelial sites, P-glycoprotein expression is localised to the apical membrane, consistent with secretory detoxifying and absorption limitation functions. The primary function of Pgp is to clear the membrane lipid bilayer of lipophilic drugs. Results from in vitro studies with human Caco-2 cells provide direct evidence for Pgp limiting drug absorption. Limitation has non-linear dependence of absorption on substrate (eg. vinblastine) concentration, increased absorption upon saturation of secretion and increased absorption upon inhibition of Pgp function, with modulators such as verapamil. The aim of this study was to investigate the effect of a known Pgp inhibitor (verapamil) and grapefruit juice components (naringenin, quercetin and bergamottin) on the transport of Rhodamine 123 across rat jejunum and to compare these results with those obtained in similar studies done in Caco-2 cells and in rat intestine (monodirectional). Verapamil, naringenin (442 µM, 662 µM and 884 µM), quercetin (73 µM, 183 µM and 292 µM) and bergamottin (12 µM, 30 µM and 48 µM) were evaluated as modulators of rhodamine 123 transport across rat jejunum using Sweetana-Grass diffusion cells. This study was done bidirectionally, with three cells measuring transport in the apical to basolateral direction (AP / BL) and three cells measuring transport in the basolateral to apical direction (BL / AP). The rate of transport was expressed as the apparent permeability coefficient (Papp) and the extent of active transport was expressed by calculating the ratio of BL/AP to AP/BL. The BL-AP/AP-BL ratio calculated for Rhodamine 123 with no modulators added was 2.31. The known modulator verapamil decreased the BL-AP/AP-BL ratio to 1.52. This was statistically significant and inhibition of active transport was clearly demonstrated. All modulators inhibited active transport. Only naringenin 884 µM, quercetin 183 µM and bergamottin 30 µM did not show a statistically significant decrease in the BL-AP/AP-BL ratio. All three components of grapefruit juice showed inhibition of active transport and should have an effect on the bioavailability of the substrates of Pgp and other active transporters. The results obtained in this study are similar to the results found in Caco-2 cells, which suggests that Sweetana-Grass diffusion method can be used for diffusion studies. / Thesis (M.Sc. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2005.

Many orally administered drugs must overcome several barriers before reaching their target site. The first major obstacle to cross is the intestinal epithelium. Although lipophilic compounds may readily diffuse across the apical plasma membrane, their subsequent passage across the basolateral membrane and into blood is by no means guaranteed. Efflux proteins located at the apical membrane, which include P-glycoprotein (P-gp, MDR1) and Multidrug Resistance-associated Protein (MRP2), may drive compounds from inside the cell back into the intestinal lumen, preventing their absorption into the blood. Intestinal P-gp is localised to the villus tip enterocytes, i.e. the main site of absorption for orally administered compounds and in close proximity to the lumen. P-gp is therefore ideally positioned to limit the absorption of compounds by driving efflux back into the lumen. Drugs may also be modified by intracellular phase I and phase II metabolizing enzymes. This process may not only render the drug ineffective, but it may also produce metabolites that are themselves substrates for P-gp and/or MRP2. Drugs that reach the blood are then passed to the liver, where they are subjected to further metabolism and biliary excretion, often by a similar system of ATP binding cassette (ABC) transporters and enzymes to that present in the intestine. Thus a synergistic relationship exists between intestinal drug metabolizing enzymes and apical efflux transporters, a partnership that proves to be a critical determinant of oral bioavailability. Aim: The aim of this study was to investigate the effect of selected methoxy flavonoids (3-methoxyflavone, 5-methoxyflavone, 6-methoxyflavone and 7- methoxyflavone) on the mean ratio of Rhodamine123 (Rho 123) transport across rat intestine (jejunum) and to investigate structure activity relationships (SAR) of the selected flavonoids with reference to inhibition of P-gp. Methods: 3-Methoxyflavone, 5- methoxyflavone, 6-methoxyflavone and 7-methoxyflavone were evaluated at a concentration of 10μM and 20μM as modulators of Rho 123 transport across rat jejunum. The Sweetana-Grass diffusion cells were used to determine the transport of Rho 123. Each modulator was studied bidirectionally with two cells measuring transport in the apical to basolateral direction (AP/BL) and two cells measuring transport in the basolateral to apical direction (BUAP). The rate of transport was expressed as the apparent permeability coefficient (Papp)and the extent of active transport was expressed by calculating the ratio of BUAP to AP/BL. Each modulators Papp ratio was then compared with that of the control. Results: 3-Methoxyflavone decreased the Papp ratio from 3.34 (control) to 1.66 (10μM) and 1.33 (20μM) and showed statistical significant differences. 7-Methoxyflavone decreased the Papp ratio to 1.94 (10μM) and 1.55 (20μM) but only showed a statistical significant difference at 10μM. 5- Methoxyflavone decreased the Papp ratio to 2.41 (10μM) and 1.71 (20μM) and 6- methoxyflavone decreased the Papp to 3.03 (10μM) and 2.49 (20μM). Both 5- and 6- methoxyflavone showed no statistical significant differences from the control. The structure activity relationships with reference to P-gp inhibition clearly indicated that the C3 and C7 positioning of the methoxy-group on the A ring played a major role in the inhibition of Rho 123 transport. Conclusion: All the selected modulators showed inhibition of Rho 123 transport across the jejunum. This should affect the bioavailability of the substrates of P-gp and other active transporters. In summary, this study describe the inhibitory interaction of selected flavonoids with P-gp. Structure activity relationships were identified describing the inhibitory potency of the flavonoids based on methoxy groups positioning. The inhibitory potency results were 3-methoxyflavone > 7- methoxyflavone > 5-methoxyflavone> 6-methoxyflavone / Thesis (M.Sc. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2005.

Background: Multidrug resistance (MDR) is resistance of cancer cells to multiple classes of chemotherapeutic drugs that can be structurally unrelated. MDR involves altered membrane transport that results in a lower cell concentration of cytotoxic drugs which plays an important role during cancer treatment. P-glycoprotein (Pgp) is localised at the apical surface of epithelial cell in the intestine and it functions as a biological barrier by extruding toxic substances and xenobiotics out of cells (Lin, 2003:54). The ATP-binding-cassette superfamily is a rapidly growing group of membrane transport proteins and are involved in diverse physiological processes which include antigen presentation, drug efflux from cancer cells, bacterial nutrient uptake and cystic fibrosis (Germann, 1996:928; Kerr, 2002:47). A number of drugs have been identified which are able to reverse the effects of Pgp, multidrug resistance protein (MRPI) and their associated proteins on multidrug resistance. The first MDR modulators discovered and studied during clinical trials were associated with definite pharmacological actions, but the doses required to overcome MDR were associated with the occurrence of unacceptable side effects. As a consequence, more attention has been given to the development of modulators with proper potency, selectivity and pharmacokinetic characteristics that it can be used at a lower dose. Several novel MDR reversing agents (also known as chemosensitisers) are currently undergoing clinical evaluation for the treatment of resistant tumours (Teodori et al., 2002:385). Aim: The aim of this study was to investigate the effect of selected flavonoids (morin, galangin, kaempferol and quercetin) at two different concentrations (10 μM and 20 μM) on the transport of a known Pgp substrate, Rhodamine 123 (Rho 123) across rat intestine (jejunum) and to investigate structure activity relationships (SAR) of the selected flavonoids with reference to the inhibition of Pgp. Methods: Morin, galangin, kaempferol and quercetin were evaluated as potential modulators of Rho 123 transport, each at a concentration of 10 μM and 20 μM across rat jejunum using Sweetana-Grass diffusion cells. This study was done bidirectionally, with two cells measuring transport in the apical to basolateral direction (AP-BL) and two cells measuring transport in the basolateral to apical direction (BL-AP). The rate of transport was expressed as the apparent permeability coefficient (Pap,) and the extent of active transport was expressed by calculating the ratio of BL-AP to AP-BL. Results: The BL-AP to AP-BL ratio calculated for Rho 123 with no modulators added was 3.29. Morin decreased the BL-AP to AP-BL ratio to 1.88 at a concentration of 10 μM and to 1.49 at a concentration of 20 μM. Galangin decreased the BL-AP to AP-BL ratio to 1.60 at a concentration of 20 μM. These two flavonoids showed statistically significant results and inhibition of active transport were clearly demonstrated. However, the other flavonoids inhibited active transport of Rho 123 but according to statistical analysis, the results were not significantly different. The two different concentrations (10 μM and 20 μM) indicated that galangin, kaempferol and quercetin showed practically significant differences according to the effect sizes. Morin, however, did not show any practically significant differences at the different concentrations. Regarding .the SAR, it was shown by Boumendjel and co-workers (2002:512) that the presence of a 5-hydroxyl group and a 3-hydroxyl group as well as the C2-C3 double bond are required for high potency binding to the nucleotide binding domain (NBD) of Pgp. All the flavonoids tested had the above-mentioned characteristics. Conclusion: All the selected flavonoids showed inhibition of active transport of Rho 123 and should have an effect on the bioavailability of the substrates of Pgp and other active transporters. This study described the inhibitory interaction of selected flavonoids on Pgp activity. Practical significant differences between the same modulator at different concentrations were also observed. Structure activity relationships were identified describing the inhibitory potency of the flavonoids based on hydroxyl group positioning / Thesis (M.Sc. (Pharmaceutics))--North-West University, Potchefstroom Campus, 2005.

Selective antiretroviral! (ARV) drugs are primarily metabolized by cytochrome P450 (CYP) enzymes, characteristically predisposed to variation, and are therefore primarily responsible for ARV pharmacokinetic variability and associated drug interactions. For the majority of ARV drugs, the therapeutic window is narrow and imminent toxicities due to CYP inhibition or sub-therapeutic drug levels as a result of CYP induction is inevitable. Animals provide a metabolism replica to conduct detailed investigations. We endeavored to establish a rat model to screen for variability in metabolism of selective ARV drugs responsible for treatment failure and drug interactions, over time in the liver and serum. Male Sprague-Dawley rats (n = 24) were divided into 6 groups: methylcellulose, 160mg/kg/day (n = 24) (control); efavirenz, 160mg/kg/day (n = 18); ritonavir, 20 mg/kg/day (n = 18); ritonavir, 20 mg/kg/day and verapamil 5 mg/kg/day (n = 18); Kaletra® (ritonavir/lopinavir), 20 mg/kg/day, (n = 18); Kaletra® (ritonavir/lopinavir), 20 mg/kg/day and verapamil 5 mg/kg/day (n = 18). Treatment duration varied from one day (single dose), 7 or 21 days. Blood samples were collected after decapitation on days 1, 7 and 21. A sensitive and rapid liquid chromatograph (LC) interfaced to a quadrupoie mass spectrometer (MS) and coupled with electrospray ionization (ESI) method was employed for the blood sample determinations. One single injection was required to simultaneously quantify efavirenz, lopinavir and ritonavir within the linear concentration range of 78 - 5000 ng/ml. Efavirenz blood levels increased statistically significantly (p < 0.05) from day 1 to day 21 with distinct steady state achievement prior to day 7. The levels of ritonavir increased statistically significantly (p < 0.05) from day 7 to 21 when administered alone and statistically significantly (p < 0.01) from day 1 to 21 when administered as the ritonavir/lopinavir combination. The levels of lopinavir also increased statistically significantly (p<0.01) from day 1 and 21 in the ritonavir/lopinavir combination. However, the inclusion of a P-glycoprotein inhibitor, verapamil, increased both the ritonavir (administered alone) and lopinavir blood levels significantly (p < 0.05) at day 1. The ritonavir levels were also significantly increased on day 21 (p < 0.05). When verapamil was added to the ritonavir/lopinavir combination the levels of ritonavir increased statistically significantly (p < 0.01) from day 1 to 21. A rat model can be used to detect changes in metabolism over time as measured by blood levels. The influence of drug interactions, such as verapamil, on ARV drug metabolism can be investigated by this model. These results will be substantiated by PCR liver results in the future. / Thesis (M.Sc. (Pharmacology))--North-West University, Potchefstroom Campus, 2009.

INTRODUCTION: The blood brain barrier (BBB), formed by the brain capillary endothelial cells separating the blood from the brain. Furthermore, the brain endothelial cells also express numerous transporter systems which help regulate and maintain the brain microenvironment. The protective function of the BBB and their transporter systems under pathological disease states, including brain tumor, can be an obstacle for the entry of therapeutic agents to the brain. OBJECTIVES: The current study set out to characterize brain tumor-induced alterations of the BBB of a mouse brain tumor model. Studies were performed to address changes in BBB permeability to P-gp dependent solutes using Rhodamine (R800). Furthermore, the use of lysophosphatidic acid (LPA) to modulate BBB permeability was also examined in healthy mice and tumor-bearing mice. METHODS: Tumors were induced by injecting Lewis Lung carcinoma (3LL) cells into the right hemisphere of female Balb/c mice. Changes in BBB permeability were assessed at various stages of tumor development, using both gadolinium contrast-enhanced agent (Gad) and 3H-mannitol. Functional activity of P-gp in the BBB was examined in adult mice following i.v. injection of R800 in the presence and absence of GF120918 (a P-gp inhibitor). Alterations in BBB permeability were characterized in healthy and tumor-bearing mice using a small (Gad) and large (IRdye800cw PEG) vascular permeability agent as well as R800 (changes in P-gp mediated permeability). RESULTS: Median mouse survival following 3LL injection was 17 days. The BBB was largely intact during tumor development with disruptions observed at the later stages of tumor development as indicated by Gad permeability. By inhibiting the function of P-gp with GF120918, the distribution of R800 in the brain increased by 4-fold. The enhancement effect of LPA on BBB permeability occurs within 3-6 minutes of injection with the barrier being restored back to its normal function within 20 minutes. Furthermore, an increased in brain penetration of IRdye800ce PEG and R800 were observed following LPA injection in both healthy and tumo-bearing mice. CONCLUSION: These studies provide the initial proof of concept for the use of BBB modulators including LPA and GF120918 to enhance drug delivery to the brain and the tumor sites.

Selective antiretroviral! (ARV) drugs are primarily metabolized by cytochrome P450 (CYP) enzymes, characteristically predisposed to variation, and are therefore primarily responsible for ARV pharmacokinetic variability and associated drug interactions. For the majority of ARV drugs, the therapeutic window is narrow and imminent toxicities due to CYP inhibition or sub-therapeutic drug levels as a result of CYP induction is inevitable. Animals provide a metabolism replica to conduct detailed investigations. We endeavored to establish a rat model to screen for variability in metabolism of selective ARV drugs responsible for treatment failure and drug interactions, over time in the liver and serum. Male Sprague-Dawley rats (n = 24) were divided into 6 groups: methylcellulose, 160mg/kg/day (n = 24) (control); efavirenz, 160mg/kg/day (n = 18); ritonavir, 20 mg/kg/day (n = 18); ritonavir, 20 mg/kg/day and verapamil 5 mg/kg/day (n = 18); Kaletra® (ritonavir/lopinavir), 20 mg/kg/day, (n = 18); Kaletra® (ritonavir/lopinavir), 20 mg/kg/day and verapamil 5 mg/kg/day (n = 18). Treatment duration varied from one day (single dose), 7 or 21 days. Blood samples were collected after decapitation on days 1, 7 and 21. A sensitive and rapid liquid chromatograph (LC) interfaced to a quadrupoie mass spectrometer (MS) and coupled with electrospray ionization (ESI) method was employed for the blood sample determinations. One single injection was required to simultaneously quantify efavirenz, lopinavir and ritonavir within the linear concentration range of 78 - 5000 ng/ml. Efavirenz blood levels increased statistically significantly (p < 0.05) from day 1 to day 21 with distinct steady state achievement prior to day 7. The levels of ritonavir increased statistically significantly (p < 0.05) from day 7 to 21 when administered alone and statistically significantly (p < 0.01) from day 1 to 21 when administered as the ritonavir/lopinavir combination. The levels of lopinavir also increased statistically significantly (p<0.01) from day 1 and 21 in the ritonavir/lopinavir combination. However, the inclusion of a P-glycoprotein inhibitor, verapamil, increased both the ritonavir (administered alone) and lopinavir blood levels significantly (p < 0.05) at day 1. The ritonavir levels were also significantly increased on day 21 (p < 0.05). When verapamil was added to the ritonavir/lopinavir combination the levels of ritonavir increased statistically significantly (p < 0.01) from day 1 to 21. A rat model can be used to detect changes in metabolism over time as measured by blood levels. The influence of drug interactions, such as verapamil, on ARV drug metabolism can be investigated by this model. These results will be substantiated by PCR liver results in the future. / Thesis (M.Sc. (Pharmacology))--North-West University, Potchefstroom Campus, 2009.

The blood-brain barrier (BBB) controls the movement of substances into and out of the brain. The tight junctions between endothelial cells and energy dependent transporters in the BBB influence rate and extent of drug distribution to the brain. The aim of this thesis was to study different methodological and pharmacokinetic aspects of drug transport at the BBB by characterizing possible active uptake and drug-drug interactions. Therefore, advanced tools for data acquisition and analysis were applied. The role of BBB transport in early drug development, with particular emphasis on in vitro-in vivo comparisons and species differences, was also investigated. Microdialysis in rats was used to study the BBB pharmacokinetics of oxymorphone, diphenhydramine (DPHM), oxycodone and morphine. Oxymorphone, DPHM and verapamil were all found to be actively taken up at the BBB, with brain to blood unbound drug ratios of 2, 5 and 2, respectively. The effect profile for oxycodone was successfully described using the modified M3 method for censored observations. In vitro experiments indicated a competitive interaction between DPHM and oxycodone on active uptake transport to the brain. No such interaction was observed in vivo due to much lower unbound concentrations achieved, compared with the in vitro Ki values. Active uptake of morphine at the BBB was not demonstrated even at very low concentrations as it was not possible to separate the active uptake transport process from active efflux by decreasing the morphine concentration. Mice carrying the human P-gp gene (hMDR1) were used to evaluate possible species differences in P-gp function. Differences were evident between the hMDR1 and normal mice in BBB penetration of various P-gp substrates and in the effect of blockers on P-gp function. Quantitative measurements of P-gp expression levels at the BBB and a comparison with human data are crucial for the future use of the hMDR1 model. In conclusion, this thesis reports active uptake of oxymorphone, DPHM and verapamil at the BBB. In vivo interaction of DPHM and oxycodone at the BBB was found not to be significant at therapeutic drug concentrations. Furthermore species differences were found between human and mouse P-gp function at the BBB.

Thesis (Ph. D.)--Cornell University, May, 1997. / Includes bibliographical references (leaves 170-181).

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 127-141).