Biometric and physiological factors in human ocular perfusion

By addressing the vascular features that characterise myopia, this thesis aims to provide an understanding of the early structural changes associated with human myopia and the progression to co-morbidity with age. This thesis addresses three main areas of study: 1. Ocular perfusion features and autoregulatory mechanisms in human myopia; 2. Choroidal thickness at the macular area of myopic eyes; 3. Effect of chronic smoking on the ocular haemodynamics and autoregulation. This thesis demonstrated a reduced resting ocular pulse amplitude and retrobulbar blood flow in human myopia, associated with an apparent oversensitivity to the vasodilatory effects of hypercapnia, which may be due to anatomical differences in the volume of the vessel beds. In young smokers, normal resting state vascular characteristics were present; however there also appeared to be increased reactivity to hypercapnia, possibly due to relative chronic hypoxia. The systemic circulation in myopes and smokers over-reacted similarly to hypercapnia suggesting that physiologic differences are not confined to the eye. Age also showed a negative effect on autoregulatory capacity in otherwise normal eyes. Collectively, these findings suggest that myopes and smokers require greater autoregulatory capacity to maintain appropriate oxygenation of retinal tissue, and since the capacity for such regulation reduces with age, these groups are at greater risk of insufficient autoregulation and relative hypoxia with age.

Design of films for oral dosage formulations

A novel method for tablet coating was studied where a thin polymer film was cast (pre-formed film), dried and applied as a coating hence eliminating the need for using any solvent during the actual coating process. A pre-formed film is initially heating to a temperature where it becomes flexible, a vacuum is applied and the film is then pulled around the tablet. The proposed films (gelatine or cellulose-based) were characterised in terms of their dissolution, swelling, mechanical and thermal properties prior to using them in the novel coating process; selected films were then coated onto tablets containing paracetamol or ibuprofen and the effect of the film on the subsequent dissolution was evaluated. It was found that the pre-formed films could be designed to be fast dissolving and mechanically strong to withstand the stress from the coating process. Also metoclopramide was incorporated in a gelatine film-coating formulation which was then successfully coated on paracetamol-containing core. Gelatin-based films were found to be successful in the novel coating process therefore to be suitable as finished coatings for immediate release dosage forms. Orally disintegrating dosage forms have been identified as a favourable dosage form due to the following reasons: fast onset of drug release, easy to use, not painful and possible increase of amount absorbed to systemic circulation. Selected films formulated for coating studies were also successfully formulated to contain active ingredient suitable for orally disintegrating dosage form; cellulose-based naratriptan-films were studied as orally disintegrating dosage forms of where the effect of formulation on the film properties was studied. It was found that strength of the film can affect the dissolution of the film but it may be the inclusion of specific excipients in the formulation which affect the penetration of the drug through mucosa.

Primary open-angle glaucoma vs normal-tension glaucoma:the vascular perspective

Objective: To compare and contrast the presence of ocular and systemic vascular function in newly diagnosed and previously untreated primary open angle glaucoma (POAG) and normal tension glaucoma (NTG) patients with comparable, early stage, functional loss. Methods: The systemic vascular function of 19 POAG patients, 19 NTG patients and 20 healthy controls was assessed by means of 24 hour ambulatory blood pressure (ABPM), peripheral pulse wave analysis (PWA) and carotid intima-media thickness (IMT). Retinal vascular reactivity to flicker light was assessed using dynamic retinal vessel analysis (DVA,IMEDOS, GmbH, Jena, Germany). Results: When compared to normal controls, both POAG and NTG patients exhibited similarly increased nocturnal systemic blood pressure variability (p=0.011); peripheral arterial stiffness (p=0.015), carotid IMT (p=0.040) and reduced ocular perfusion pressure (OPP) (p<0.001). Furthermore, on DVA analysis, both groups of glaucoma patients also exhibited steeper retinal arterial constriction slopes (slope AC) following cessation of flicker (p=0.007) and a similarly increased fluctuation in arterial and venous baseline diameter (p=0.008 and p=0.009 respectively) in comparison to controls. Conclusion: POAG and NTG patients exhibit similar alterations in both ocular and systemic circulation at the early stages of their disease process. This highlights not only the importance of considering vascular risk factors in both conditions, but also raises questions about the current separation of the two conditions into completely distinct clinical entities.

Hypercholesterolaemia-induced oxidative stress at the blood-brain barrier

Blood cholesterol levels are not consistently elevated in subjectswith age-related cognitive decline, although epidemiological studies suggest that Alzheimer's disease and cardiovascular diseases share common risk factors. These include the presence of an unusual genetic variant, the APOE4 (apolipoprotein E4) allele, which modulates LDL (low-density lipoproteins) metabolism, increases free radical formation and reduces plasma antioxidant concentrations. Together, these risk factors support a mechanism for increased LDL circulation time and free radical modification of LDL. Plasma oxycholesterols, hydroxylated metabolites of cholesterol, are carried by oxidized LDL, and elevated lipids in mid-life are associated with increased longterm risk of dementia. Although brain cholesterol metabolism is segregated from the systemic circulation, during oxidative stress, plasma oxycholesterols could have damaging effects on BBB (blood-brain barrier) function and consequently on neuronal cells. Cholesterol-lowering drugs such as statins may prevent the modifications to LDL in mid-life and might show beneficial effects in later life. © The Authors Journal compilation © 2014 Biochemical Society.

Physiologically based pharmacokinetic modelling in drug delivery

The application of pharmacokinetic modelling within the drug development field essentially allows one to develop a quantitative description of the temporal behaviour of a compound of interest at a tissue/organ level, by identifying and defining relationships between a dose of a drug and dependent variables. In order to understand and characterise the pharmacokinetics of a drug, it is often helpful to employ pharmacokinetic modelling using empirical or mechanistic approaches. Pharmacokinetic models can be developed within mathematical and statistical commercial software such as MATLAB using traditional mathematical and computation coding, or by using the Simbiology Toolbox available within MATLAB for a graphical user interface approach to developing pharmacokinetic (PBPK) models. For formulations dosed orally, a prerequisite for clinical activity is the entry of the drug into the systemic circulation.

8-Hydroxydeoxyguanosine. A marker of oxidative DNA damage in systemic lupus erythematosus

8-Hydroxydeoxyguanosine (80HDG) is a specific marker of oxidative damage to DNA. We have observed that patients with SLE (systemic lupus erythematosus), have undetectable levels of urinary 80HDG by HPLC. Further analysis by GC-MS confirmed that levels of 80HDG in SLE urine were 10(3)-fold lower than in an age- and sex-matched control group. Experiments utilising cultures of SLE and normal lymphocytes exposed to H2O2 confirmed the impaired ability of SLE lymphocytes to repair 80HDG. We subsequently observed in SLE patients that 80HDG had accumulated in low molecular weight DNA associated with circulating immune complexes. We suggest that oxygen radicals may induce pathology in SLE by maintaining the presence of an antigenic form of DNA in the circulation.

First-line therapy with latanoprost 0.005% results in improved ocular circulation in newly diagnosed primary open-angle glaucoma patients: a prospective, 6-month, open-label study

Purpose To evaluate the effect of latanoprost 0.005% on the optic nerve head (ONH) and retinal circulation of newly diagnosed and previously untreated primary open-angle glaucoma (POAG) patients. Methods Twenty-two newly diagnosed and previously untreated POAG patients (mean age±SD: 68.38±11.92 years) were included in this longitudinal open-label study. Patients were treated with latanoprost 0.005% once a day. Intraocular pressure (IOP), systemic blood pressure (BP), mean ocular perfusion pressure (MOPP), and ocular perfusion parameters ‘volume’, ‘velocity’, and ‘flow’ measured at the optic nerve head (ONH) and retina by means of Heidelberg Retina Flowmeter system were evaluated during a 6-month follow-up period. Results Treatment with latanoprost 0.005% resulted in a significant decrease in IOP (P<0.0001) and increase in MOPP (P<0.0001). After correcting for changes in MOPP, the blood velocity measured at the ONH level was significantly higher after 6 months of treatment than at baseline (P=0.0310). In addition, blood volume and flow measured at the peripapillary retina level improved after 3 and 6 months of treatment (P=0.0170; P=0.0260, and P=0.0170; P=0.0240 respectively). Conclusion Previously untreated POAG patients exhibit reduced IOP, increased MOPP and improved ocular perfusion at the ONH and retina levels when treated with Latanoprost 0.005%. These effects could be beneficial for glaucoma patients suffering from ocular vascular dysregulation.

Retinal and systemic vascular function in health and disease: the effect of smoking and coronary artery disease

The purpose of the following studies was to explore the effect of systemic vascular and endothelial dysfunction upon the ocular circulation and functionality of the retina. There are 6 principal sections to the present work. Retinal vessel activity in smokers and non-smokers: the principal findings of this work were: chronic smoking affects retinal vessel motion at baseline and during stimulation with flickering light; chronic smoking leads to a vaso-constrictory shift in retinal arteriolar reactivity to flicker; retinal arteriolar elasticity is decreased in chronic smokers. The effect of acute smoking on retinal vessel dynamics in smokers and non-smokers: the principal finding of this work was that retinal reactivity in chronic smokers is blunted when exposed to clicker light provocation immediately after smoking one cigarette. Ocular blood flow in coronary artery disease: The principal findings of this work were: retrobulbar and retinal blood flow is preserved in CAD patients, despite a change pulse wave transmission; arterial retinal response to flickering light provocation is significantly delayed in CAD patients; retinal venular diameters are significantly dilated in CAD patients. Autonomic nervous system function and peripheral circulation in CAD: The principal findings in this work were: CAD patients demonstrate a sympathetic overdrive during a 24 period; a delay in peripheral vascular reactivity (nail-fold capillaries) as observed in patients suffering from CAD could be caused by either arteriosclerotic changes of the vascular walls or due to systemic haemodynamic changes. Visual function in CAD: The principal findings in this work were: overall visual function in CAD patients is preserved, despite a decrease in contrast sensitivity; applying a filtering technique selecting those with greater coefficient of variance which in turn represents a decrease in reliability, some patients appear to have an impaired visual function as assessed using FDT visual field evaluation. Multiple functional, structural and biochemical vascular endothelial dysfunctions in patients suffering from CAD: relationships and possible implications: The principal findings of this work were: BMI significantly correlated with vWF (a marker of endothelial function) in CAD patients. Retinal vascular reactivity showed a significant correlation with peripheral reactivity parameters in controls which lacked in the CAD group and could reflect a loss in vascular endothelial integrity; visual field parameters as assessed by frequency doubling technology were strongly related with systemic vascular elasticity (ambulatory arterial stiffness index) in controls but not CAD patients.