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How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion.

Kell DB, Oliver SG - Front Pharmacol (2014)

Bottom Line: One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest.One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose "natural" biological roles, and substrates are based in intermediary metabolism.Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, The University of Manchester Manchester, UK ; Manchester Institute of Biotechnology, The University of Manchester Manchester, UK.

ABSTRACT
One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose "natural" biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

No MeSH data available.


Related in: MedlinePlus

A typical biomembrane drawn roughly to scale, indicating the typical protein:lipid mass ratio and the possible means by which a small molecule drug (atorvastatin, also drawn to scale) might cross, including (A) bilayer lipoidal diffusion through bilayer areas nominally unaffected by the presence of proteins, or (B) by hitchhiking a lift on transporters (Schlessinger et al., 2013b) normally present for the purposes of intermediary metabolism. At issue is the question of whether there is any untrammeled bilayer that might let the atorvastatin leak across, and whether biophysical properties such as log D or log P can account for this. Typically atorvastatin is in fact transported by a bile acid transporter known as OATP1B1 (SLCO or SLC21 family) (e.g., Hagenbuch and Stieger, 2013; Higgins et al., 2014).
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Figure 5: A typical biomembrane drawn roughly to scale, indicating the typical protein:lipid mass ratio and the possible means by which a small molecule drug (atorvastatin, also drawn to scale) might cross, including (A) bilayer lipoidal diffusion through bilayer areas nominally unaffected by the presence of proteins, or (B) by hitchhiking a lift on transporters (Schlessinger et al., 2013b) normally present for the purposes of intermediary metabolism. At issue is the question of whether there is any untrammeled bilayer that might let the atorvastatin leak across, and whether biophysical properties such as log D or log P can account for this. Typically atorvastatin is in fact transported by a bile acid transporter known as OATP1B1 (SLCO or SLC21 family) (e.g., Hagenbuch and Stieger, 2013; Higgins et al., 2014).

Mentions: Statement: “The mass ratio of protein:lipid in vivo (1/1 to 3/1) affects the transport properties of lipids. Artificial membranes do not model biological membranes, owing to the high protein content in vivo. Response: These ratios include the cytoplasmic and exoplasmic portions of membrane protein mass, not just the relevant transmembrane fraction. The lipid:protein molar ratio is estimated as 40:1, making lipid an important portion of the membrane exposed to drug molecules. A further refined consideration would take into account the relative cross-sectional area at the membrane surface of the 40 phospholipid molecules to one typical protein. The lipid surface area would still be significantly greater than that of the transporter protein.” Counter: The surface area per se is not the question. What matters is the extent to which the presence of high amounts of protein in a cell membrane (Dupuy and Engelman, 2008), often binding specific lipids (Laganowsky et al., 2014) (including cholesterol; Song et al., 2014) and certainly altering their organization (Mitra et al., 2004; Engelman, 2005; Mclaughlin and Murray, 2005; Beswick et al., 2011; Coskun and Simons, 2011; Kusumi et al., 2011; Lee, 2011a,b; Domański et al., 2012; KoldsØ and Sansom, 2012; Magalon et al., 2012; Mueller et al., 2012; Smith, 2012; Goose and Sansom, 2013; Javanainen et al., 2013; Van Der Cruijsen et al., 2013) (and vice versa; Li et al., 2012; Denning and Beckstein, 2013), alters any ability of drug molecules to cross via the lipoidal bilayer part of the membranes in which these proteins exist. This means that any direct change of lipids will also have the potential likelihood of affecting transporters, so is not of itself a discriminating experiment if transporter activities are not measured. We see an important role for molecular dynamics simulation studies here [see e.g., those of Sansom and colleagues (Stansfeld and Sansom, 2011; Stansfeld et al., 2013), of Tajkhorshid and colleages (Khalili-Araghi et al., 2009; Wang et al., 2010b; Enkavi et al., 2013; Moradi and Tajkhorshid, 2013; Shaikh et al., 2013; Han et al., 2014; Mishra et al., 2014), and of others (e.g., Gedeon et al., 2010; Skovstrup et al., 2012; Denning and Beckstein, 2013; KoldsØ et al., 2013; Schlessinger et al., 2013b)], and note that even CO2 can traverse membranes via the central pore in aquaporin (Wang et al., 2010b; Kaldenhoff et al., 2014; Li et al., 2014). It is here worth reminding readers of what membranes actually look like (in cartoon form) (Engelman, 2005), of the size of phospholipid head groups relative to the size of a drug such as atorvastatin (Figures 5A,B), and of the consequent unlikelihood of a drug floating unaided swiftly through a phospholipid bilayer in a real biomembrane.


How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion.

Kell DB, Oliver SG - Front Pharmacol (2014)

A typical biomembrane drawn roughly to scale, indicating the typical protein:lipid mass ratio and the possible means by which a small molecule drug (atorvastatin, also drawn to scale) might cross, including (A) bilayer lipoidal diffusion through bilayer areas nominally unaffected by the presence of proteins, or (B) by hitchhiking a lift on transporters (Schlessinger et al., 2013b) normally present for the purposes of intermediary metabolism. At issue is the question of whether there is any untrammeled bilayer that might let the atorvastatin leak across, and whether biophysical properties such as log D or log P can account for this. Typically atorvastatin is in fact transported by a bile acid transporter known as OATP1B1 (SLCO or SLC21 family) (e.g., Hagenbuch and Stieger, 2013; Higgins et al., 2014).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4215795&req=5

Figure 5: A typical biomembrane drawn roughly to scale, indicating the typical protein:lipid mass ratio and the possible means by which a small molecule drug (atorvastatin, also drawn to scale) might cross, including (A) bilayer lipoidal diffusion through bilayer areas nominally unaffected by the presence of proteins, or (B) by hitchhiking a lift on transporters (Schlessinger et al., 2013b) normally present for the purposes of intermediary metabolism. At issue is the question of whether there is any untrammeled bilayer that might let the atorvastatin leak across, and whether biophysical properties such as log D or log P can account for this. Typically atorvastatin is in fact transported by a bile acid transporter known as OATP1B1 (SLCO or SLC21 family) (e.g., Hagenbuch and Stieger, 2013; Higgins et al., 2014).
Mentions: Statement: “The mass ratio of protein:lipid in vivo (1/1 to 3/1) affects the transport properties of lipids. Artificial membranes do not model biological membranes, owing to the high protein content in vivo. Response: These ratios include the cytoplasmic and exoplasmic portions of membrane protein mass, not just the relevant transmembrane fraction. The lipid:protein molar ratio is estimated as 40:1, making lipid an important portion of the membrane exposed to drug molecules. A further refined consideration would take into account the relative cross-sectional area at the membrane surface of the 40 phospholipid molecules to one typical protein. The lipid surface area would still be significantly greater than that of the transporter protein.” Counter: The surface area per se is not the question. What matters is the extent to which the presence of high amounts of protein in a cell membrane (Dupuy and Engelman, 2008), often binding specific lipids (Laganowsky et al., 2014) (including cholesterol; Song et al., 2014) and certainly altering their organization (Mitra et al., 2004; Engelman, 2005; Mclaughlin and Murray, 2005; Beswick et al., 2011; Coskun and Simons, 2011; Kusumi et al., 2011; Lee, 2011a,b; Domański et al., 2012; KoldsØ and Sansom, 2012; Magalon et al., 2012; Mueller et al., 2012; Smith, 2012; Goose and Sansom, 2013; Javanainen et al., 2013; Van Der Cruijsen et al., 2013) (and vice versa; Li et al., 2012; Denning and Beckstein, 2013), alters any ability of drug molecules to cross via the lipoidal bilayer part of the membranes in which these proteins exist. This means that any direct change of lipids will also have the potential likelihood of affecting transporters, so is not of itself a discriminating experiment if transporter activities are not measured. We see an important role for molecular dynamics simulation studies here [see e.g., those of Sansom and colleagues (Stansfeld and Sansom, 2011; Stansfeld et al., 2013), of Tajkhorshid and colleages (Khalili-Araghi et al., 2009; Wang et al., 2010b; Enkavi et al., 2013; Moradi and Tajkhorshid, 2013; Shaikh et al., 2013; Han et al., 2014; Mishra et al., 2014), and of others (e.g., Gedeon et al., 2010; Skovstrup et al., 2012; Denning and Beckstein, 2013; KoldsØ et al., 2013; Schlessinger et al., 2013b)], and note that even CO2 can traverse membranes via the central pore in aquaporin (Wang et al., 2010b; Kaldenhoff et al., 2014; Li et al., 2014). It is here worth reminding readers of what membranes actually look like (in cartoon form) (Engelman, 2005), of the size of phospholipid head groups relative to the size of a drug such as atorvastatin (Figures 5A,B), and of the consequent unlikelihood of a drug floating unaided swiftly through a phospholipid bilayer in a real biomembrane.

Bottom Line: One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest.One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose "natural" biological roles, and substrates are based in intermediary metabolism.Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, The University of Manchester Manchester, UK ; Manchester Institute of Biotechnology, The University of Manchester Manchester, UK.

ABSTRACT
One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose "natural" biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

No MeSH data available.


Related in: MedlinePlus