Biopharmaceutics Flashcards
Why is oral drug delivery worth so much in the pharmaceutical market?
The oral drug delivery is predicted to reach 150 billion in 2024 and is seen so valuably in the pharmaceutical industry due to improved patient compliance in comparison to equivalent intravenous preparations. However, oral formulations are increasingly difficult to manufacturer in comparison to intravenous formulations (which ensure direct administration of the drug into the bloodstream) as in oral drug delivery the drugs must have the added ability of successfully crossing the gastro-intestinal epithelium and mucosal layer to then be absorbed into the bloodstream.
What is the focus of the oral drug delivery market?
Adapting and manipulating intravenous drug formulations to make them available as an oral formulation to improve patient compliance and cost long-term due to self-medicating rather than attending healthcare care settings for medication administration.
How do the main barriers of drug delivery differ between oral and intravenous drugs?
In oral drug delivery a major limiting factor for the efficacy of the drug is its ability to reach the bloodstream by travelling through the digestive system. This includes ensuring maximum absorption (links to the physiochemical properties of the drug) across the GI epithelium by overcoming the mucosal barrier, not degrading under the harsh acidic environments within the stomach or digestive enzymes and not being removed by rapid gastric emptying or affected by gastric motility.
Whereas with intravenous drug delivery, as discussed in the nanomedicine topics, the main concerns that can affect drug efficacy is rapid renal clearance and elimination of the drug, rapid first pass hepatic metabolism, cell like drugs can be quickly targeted for uptake into the mononuclear phagocytic system reducing ability of the drug to accumulate via the EPR effect. Enzymes such as peptidases can also degrade drugs.
Briefly list the clinical barriers associated with oral drug delivery.
Fed and fasted state variability in drug absorption
Inter and intra individual differences in oral bioavailability
Differences in gastric emptying time and GI transit time
Disease state
Describe how some of the physiochemical properties (drug related) can affect drug absorption.
Poor aqueous solubility
pH dependent solubility
Extensive ionisation at GI pH range
Extreme lipophilicity
High molecular weight
Susceptibility to pH mediated degradation
Describe how some of the physiological properties can affect drug absorption.
Mucus barrier
Diverse pH range of the GI tract (1.2-7.4)
Rapid gastric emptying
Gastric and intestinal motility
First pass metabolism
Presence of digestive enzymes
GI microflora and their secretions
Describe how some of the biopharmaceutical properties can affect drug absorption.
Poor drug permeability
Pre-systemic metabolism in the GI tract
Presence of drug efflux transporters
pH and mucosal layer thickness variations in the GI tract dependent on location
Where is mucus found in the body?
Mucus is found on any moist surface of the body to coat mucosal linings. This includes the eyes, respiratory tract, mouth, stomach, nose in addition to the intestines.
Describe the generalised function of mucus.
The main function of mucus which coats the epithelium is the physical defence it provides against pathogens as part of the innate immune system. The sticky hydrogel which prevents the penetration of pathogens into the epithelial layer initiating an immune response and causing inflammation.
Mucus also provides lubrication to its surfaces allowing the passage of food through the GI tract, movement of the eyes and to prevent tissues from drying out.
In addition to prevent pathogens into the epithelium, mucus also selectively enables the crossing of nutrients and small molecules to be absorbed from dietary sources into the intestinal epithelial cells and eventually the bloodstream.
Mucus layers that cover the intestinal epithelium in both the small and large intestine is inhabited with bacterium known as mucus-associated microbiota. This physical barrier segregates the gut microbiota and host immune system.
Describe the composition of mucus in both the small and large intestine, and its function in the intestinal tract.
In the small intestine the villi are covered in a loose mucus layer which small molecules and nutrients are able to penetrate for absorption whereas in the large intestine, due to abundance of bacterium present and hence a much higher number of goblet cells, the epithelium is covered in a outer loose mucus layer and a dense inner mucus layer where bacterium is not present. The density and the thickness of the inner mucus layer increases from the proximal to the distal colon. These mucus layers are composed of goblet secreted Mucin-2 proteins, highly O-glycosylated, which forms a net like structure. The inner layer of mucus is stratified and remains anchored to the epithelium. The inner layer is converted to the outer layer by polymerisation of MUC-2 by gut microbiota or the host. Lamina propia plasma cells produce anti-commensal immunoglobulins such as IgA which prevents invasion of bacterium into the inner mucus layer or the activation of the immune system by presence of the bacterium. The main function of mucus therefore in both the small and large intestine is the ensuring the segregation of the gut microbiota and the epithelium and immune system.
Is mucus more critical for pathogenic defence in the small or large intestine?
Arguably the physical barrier of mucus plays a more critical role in immune defence within the large intestine in comparison to the small intestine. This is because within the small intestine there are a number of chemical defences such as Paneth cells which secrete antimicrobial peptides, defending against gram positive and gram negative bacterium. Where chemical defences such as presence of Paneth cells secreting AMPs are not present in the large intestine, physical barriers such as the mucosal layer are primarily responsible for ensuring the spatial segregation of the gut microbiota and intestinal epithelia.
What are some of the clinical complications categorised by modifications in mucus or absence of it?
Conditions such as dry eye syndrome is caused by the inability of tear ducts to work and produce adequate lubrication (production of mucus) causing a gritty, itchy feeling. When there is a defect in the mucus layer in the colon this leads to colitis, normally attributed to a reduction in the number of goblet cells.
In cystic fibrosis, instead of the mucus being quite thin and slippery in order to provide lubrication, there are defect in the goblet cells causing the mucus to become thick and sticky resulting in tubes becoming plugged up in the body such as in the lungs and pancreas.
Describe the synthesis and structure of mucin.
Mucins are high–molecular weight glycoproteins that are heavily glycosylated with oligosaccharides (sugars) linked by mainly O-glycosylation (although can get some N-glycosylation) to serine or threonine in the protein backbone. This means that the glycoproteins become highly decorated with glycosylation patterns that differ considerably in different cell or tissue types of the body within the same mucin type. Both membrane bound and secreted mucins are initially synthesised in the ribosomes of the rough endoplasmic reticulum where the protein core (apomucin) is synthesised followed by some post-translational modifications such as N-glycosylation, C-mannosylation and
dimerization. These modified apomucins are packed into vesicles and transported to the cis-Golgi on microtubules where O-linked glycosylation occurs. Once this has been completed, mucins in the trans-Golgi network are packed into vesicles (if secretory) or become membrane bound.
Give an example of how the same mucin type can have different glycosylation patterns.
For example cervical mucin glycosylation (MUC5B, 5AC) the combination of glycotransferases in addition to their levels in the body are hormonally regulated and change at different points of the menstrual cycle.
And more than 100 different O-linked
oligosaccharide structures have been identified on a single colonic MUC2 protein.
What are the two structures mucins are known to form?
Secretory mucins within the mucus are said to form either linear polymer models (parallel rope like structures) with some degree of flexibility or N-terminal trimer network model where a mucin is capable of attaching at three points resulting in the formation of a ladder like structure.
Aside from mucins what are some of the other components of mucus?
Alongside mucins, mucus also consists of 95% water alongside proteoglycans, DNA, lipids, bile acids, salts etc
To what extent does particle size affect diffusion through the mucosal layer?
On one hand it appears that particle size is the dependent variable for determining the diffusion rates through the mucosal layer. The mesh pore size of approximately 10-200 nm sets a threshold for which exceeded, diffusion of nanoparticles is reduced. For example 100 nm nanoparticles were uptaken in the GI tract to a greater extent than nanoparticles 1000 nm.
However it appears that smaller particle sizes does not always correlate to the increased rate of diffusion across the mucosal layer in different parts of the body and other factors such as heterogeneous nature of the pore network and intermolecular interactions. For example, diffusion across cervical mucus was increased by larger nanoparticles and further enhanced the PEGylation of these nanoparticles. Large pores within the highly elastic mucin fibre matrix filled with low viscosity fluid enables the easy diffusion of molecules and viruses such as the HPV (55nm) through the mucus, with the potential for a similar rate of diffusion as that through water, providing absence of adhesion to that of the mucin fibres.
Does the mucus layer have some selectivity for inhibiting the diffusion of viruses?
It appears that to an extent, mucus has some selectivity to inhibiting the diffusion of viruses across its layer before penetrating the epithelium but this is largely dependent on the different types of mucus in different cell/tissue types in the body due to differing viscosities (mucin glycosylation patterns). It appears that capsid viruses such as papillomavirus and adenovirus (less than 100 nm) can diffuse readily across the mucus, as the pore size is 10-200 nm however enveloped viruses such as the herpes simplex virus with a diameter of 180 nm was completely inhibited due to potentially forming low affinity bonds to the mucins and therefore suggests there is some selectivity for the passage of substances across the mucus.
Could it be argued that at different points of the menstrual cycle, passage of HPV is more likely?
The mucins that constitute the cervical mucus appear to undergo different glycosylation patterns at different points of the menstrual cycle. Hormonal regulation is an important component as mucin composition and viscoelasticity changes during the menstrual cycle which can be attributed to increases in cervical mucus production and alterations in the mucin glycosylation making the mucus essentially more watery and hence permeable to sperm. However the lower viscosity of mucus at the point of ovulation may also make the diffusion of viruses such as HPV easier due to reduction of viscosity and hence likelihood of the virus to adhere to mucin fibres.
How should drugs be presented at the epithelium for sufficient absorption?
Essentially you want to build up a high concentration of drug at the epithelium to ensure sufficient absorption across. This is achieved by modification of the drug release profile which can enable a sufficient concentration of drug to build up, this should be ideally between 2-3 hours. The consequences of the drug release profile being within a shorter time of this is that the drug will not reach the intestinal epithelium, and instead will be released earlier in the GI tract and potentially undergo renal clearance before reaching in the intestine meaning that a concentration gradient will not be achieved. If the drug release profile is slower than this then drug concentration will be too shallow at the epithelium and again absorption will not occur.
Describe the muco-adhesive approach to drug delivery across the mucosal barrier.
Muco-adhesive drug delivery involves the interaction of the drug/carrier system with the mucin molecules within the mucus covering the epithelium. This increase in intimate contact with mucin molecules increases the residence time and concentration of drug present at the epithelium causing an increase in absorption and hence therapeutic effect. The adherence to mucin molecules occurs in two stages firstly the contact stage which involves the contact between the muco-adhesive materials and the mucus membrane resulting in its embedment within the mucus surface. Presence of moisture then initiates the consolidation stage where the swelling/gelling of muco-adhesive materials then causes the adherence to mucins, increasing their residence time.
What are the advantages and disadvantages of the muco-adhesive system?
Muco-adhesive properties increases the targeting and localisation of drug concentrations leading to an enhanced therapeutic effect with lower doses (reducing the potential for adverse effects). In addition at the epithelium, the intimate contact between the drug and mucins increasing the rate of absorption of oral dosage forms.
However there are some disadvantages of these systems including relying on the ability of the drug once embedded in the mucus membrane to diffuses across against the production, secretion, turnover and clearance of new mucus however it does ensure it no longer remains in the lumen.
Describe the mucus penetrative approach to drug delivery across the mucosal barrier.
Muco-penetration rather than an active form of drug delivery involves modification of nano-particles to prevent the muco-adhesion of drugs to the mucin fibres and hence penetration, as already small enough, into the epithelium. This involves modifications to the surface of nanoparticles such as PEGylation to avoid uptake. An example of this was shown to be beneficial in the nanoparticle formulation for vaginal drug delivery in which PEGylated drugs were shown to have higher diffusion rates than non-PEGylated nano-particles due to lack of adherence to mucin fibres leading to an improved therapeutic effect.
State the five routes of drug absorption across the intestinal epithelium.
Two mechanisms of passive diffusion:
Transcellular route, across the cell membranes
Paracellular route, through tight junctions
Transcellular route, by utilisation and uptake of active transporters present on the cell membrane
Lipid absorption via the formation of spontaneous micelles under the presence of bile salts
Particulate absorption via the gut-associated lymphatic tissue
Describe the mechanism of passive transcellular absorption.
Drug molecules that undergo absorption from passive transcellular absorption is initiated by achieving a high drug concentration and hence substantial concentration gradient across the epithelial membrane. A high drug concentration gradient within the lumen and a low drug concentration within the epithelium will cause drug molecules to partition from the apical to basolateral membrane and deposit or diffuse (under Fick’s Law) across the cell and then partition into the endothelium.
Which types of drugs are capable of undergoing passive transcellular absorption?
Most drug molecules are capable of undergoing passive diffusion across the epithelium however these drugs must possess certain properties:
Drugs larger than 100-200 Da are usually unable to cross the membrane, the smaller the drug the easier the diffusion is across the membrane
Drugs must have a reasonably high lipophilicity (Log P) in order to be absorbed into the phospholipid bilayer, however a Log P too high prevents the drug partitioning into the endothelium on the other surface membrane and will remain within the cell membrane
Ionised and charged drug molecules are impermeable to the cell membrane and so so must present in their unionised form at the epithelium which is lipid soluble.
