La régulation transcriptionnelle des eucaryotes supérieurs est un processus hautement contrôlé du point de vue spatial et temporel lors du développement, ou en réaction à l’environnement. La transcription ciblée des gènes codant requiert l’assemblage d’un complexe de pré-initiation (PIC) aux promoteurs comprenant l’ARN Polymérase (Pol) II et les facteurs généraux de transcription (GTFs) et dépend de la médiation d’un signal par les facteurs activateurs de transcription (TFs). Les années récentes ont montré que la transition de l’initiation vers l’élongation productive de la transcription représente une étape clé de la régulation de l’expression des gènes. Ce processus est également contrôlé par la structure de la chromatine, les modifications d’histones et par la présence d’éléments cis-régulateurs tels que les ‘enhancers’ ou les ‘silencers’. Au cours de ma thèse, nous avons entrepris de décrypter les mécanismes de régulation transcriptionnelle impliqués dans les étapes du développement lymphocytaire. Nous avons essentiellement travaillé sur des thymocytes primaires murins isolés au stade de différenciation double positif (DP, CD4+/CD8+) pour lequel de nombreuses séquences de type ‘enhancers’ ont été caractérisées dans la littérature scientifique. Nous avons également utilisé des lymphocytes B humains (Raji) immortalisés pour certaines des expériences impliquant des manipulation génétiques complexes permettant l’étude de mutants du domaine carboxy-terminal (CTD) de Pol II. En couplant des approches d’analyse à l’échelle du génome au séquençage à haut-débit, nous avons établi des cartographies fines de la localisation de Pol II, des GTFs, des TFs,de modifications d’histones (ChIP-Seq) et de nucléosomes (MNase-seq) ainsi que la caractérisation de populations variées d’ARN par RNA-seq. Nos principaux résultats ont révélé (i) l’assemblage du PIC et la transcription des enhancers tissus-spécifiques, (ii) l’existence de plateforme d’initiation de la transcription (TIPs) aux enhancers et aux promoteurs tissus-spécifique, (ii) que le contenu en GC représente l’un des principaux éléments promoteurs mammifères en permettant une ouverture transcription-indépendante de la chromatine, (iv) l’importance d’une nouvelle modification post-traductionnelle du domaine CTD de Pol II pour la progression de l’enzyme en élongation et finalement (v) que la modification de l’histone H3 sur le résidu K36 methylé corrèle avec l’épissage des transcrits Pol II. Globalement, les résultats les plus important de ce manuscrit consistent dans la mise en évidence de la transcription des enhancers comme caractérisant l’expression des gènes tissus-spécifiques et dans l’importance des ilots CpG comme éléments promoteurs mammifères permettant la formation d’une structure ouverte de la chromatine. / Transcriptional regulation in higher eukaryotes resembles a tightly controlled temporal and spatial process, as exemplified during development or an organism’s response to environmental stimuli. Directed transcription requires the assembly of the preinitiation complex (PIC) at the promoter of protein-coding genes, including RNA Polymerase (Pol) II and the general transcription factors (GTFs), mediated by activating transcription factors (TFs). Several rate-limiting steps further control the progression of Pol II initiation to productive elongation of the gene. This process is further controlled by chromatin structure, histone modifications as well as cis-regulatory elements, such as enhancers or silencers. We set out to decipher some of these regulatory mechanisms during the tightly controlled process of lymphocyte development. Our work primarily made use of primary mouse thymocytes in CD4+/CD8+ double positive (DP, CD4+/CD8+) stage during T-cell development. To our advantage, many developmentally important cis-regulatory regions are well characterized in this cell population. For genetic manipulations, we made use of the Raji B-cell lymphoma cell-line. Using high throughput genome-wide approaches based on next generation sequencing (NGS), we performed both localization studies of Pol II, GTFs, TFs, histone modifying enzymes, histone modifications and nucleosomes as well as deep-sequencing of different RNA transcript populations. In summary, we find that (i) PICs assemble at tissue-specific enhancers leading to local transcription, (ii) large transcription initiation platforms (TIPs) at tissue-specific promoters and enhancers exist, which correlate with high CG-content of the DNA and transcription factor binding sites (TFBS), (iii) GC-content regulates the nucleosomal structure and initiation, including directionality, at promoters, (iv) Pol II is phosphorylated at a new residue of it C-terminal domain (CTD) in the 3’ regions of genes and (v) splicing events can influence the chromatin structure. Altogether, these results show that PIC formation at and transcription of enhancers are important for the regulation of T-cell target genes, that CpG islands represent important if not the major regulatory promoter element in mammals guiding tissue-specific gene expression and nucleosome structure, as well as novel mechanisms of Pol II elongation and the effect on chromatin structure.

La différenciation cellulaire implique une régulation transcriptionnelle coordonnée et finement contrôlée qui passe par le recrutement de facteurs de transcription (FT) cellules-spécifiques sur des régions génomiques régulatrices appelées enhancers. Parmi ces FT, des protéines nommées « facteurs pionniers » (FP) lient la chromatine condensée et favorisent la transition des enhancers d’un état inactif vers un état « préparé » (étape de « priming »), facilitant ainsi la fixation d’autres FT et permettant l’activation de ces régions. L’engagement vers un lignage cellulaire particulier est donc associé à l’engagement des FP au niveau d’enhancers dont la structure chromatinienne subit des changements architecturaux associés à la mise en place de marques spécifiques. Celles-ci incluent, la monométhylation de la lysine 4 de l’histone H3 (H3K4me1), l’acétylation de la lysine 27 de l’histone H3 (H3K27ac) ou encore des modifications des résidus cytosine (5-méthylcytosine, 5mC ; 5-hydroxyméthylcytosine, 5hmC). La 5hmC est un intermédiaire de la voie de déméthylation active : elle résulte de l’oxydation de la 5mC par les enzymes « Ten Elven Translocation » (TET) et peut être à son tour oxydée en 5-formylcytosine (5fC) et 5-carboxylcytosine (5caC) qui sont ensuite remplacées par des cytosines via l’intervention du système « Base Excision Repair ». Cependant, du fait de sa stabilité et de sa capacité à lier des protéines particulières, la 5hmC pourrait également jouer un rôle spécifique. De précédents travaux ont d’ores et déjà mis en évidence un lien entre le recrutement des FP et les modifications des cytosines. Néanmoins, l’implication des processus de méthylation/déméthylation dans la régulation spatio-temporelle des étapes de « priming » et d’activation des enhancers n’a pas encore été caractérisée. Dans ce contexte, l’objectif de cette étude à été de définir le rôle des modifications de cytosines (5mC et 5hmC) lors de l’activation des enhancers liés par des FP. Pour ceci, nous avons analysé d’une part, l’implication des processus de méthylation et déméthylation des cytosines sur le « priming » et l’activation des enhancers, en utilisant des inhibiteurs des ADN méthyltransférases ou des enzymes TET. D’autre part, nous avons entrepris d’identifier les dynamiques de « priming » et d’activation des enhancers à l’échelle du génome au cours de la différenciation neurale, en lien avec la présence de la 5hmC. Les résultats obtenus nous ont notamment permis de proposer un schéma d’activation des enhancers dans lequel les dynamiques de méthylation/déméthylation de l'ADN jouent un rôle essentiel dans la structuration de la chromatine. / Cell differentiation relies on a coordinated and finely regulated transcriptional regulation involving the recruitment of cell-type transcription factors (TFs) on genomic regions called enhancers. Some of these TFs, named pioneer factors (PFs), are able to bind to condensed chromatin and favour enhancer transition from an inactive to a primed state, thus facilitating the binding of other TFs and enhancer activation. Therefore, lineage commitment is associated to the engagement of PFs at enhancers where the chromatin structure undergoes architectural modifications related to the set up of specific marks. These include, the monomethylation of the lysine 4 of the histone H3 (H3K4me1), the acetylation of the lysine 27 of the histone H3 (H3K27ac) or cytosine modifications (5-methylcytosine, 5mC; 5-hydroxymethylcytosine, 5hmC). The 5hmC base is an intermediate in the process of active demethylation coming from the oxidation of the 5mC by the Ten Elven Translocation (TET) enzymes and can itself be further oxidized in 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC), two bases which are then replaced by cytosines through the Base Excision Repair mechanism. Nevertheless, due to its stability and its ability to bind some specific proteins, 5hmC might also play specific roles. Previous works already highlighted a link between the recruitment of PFs and cytosine modifications. However, the involvement of the methylation/demethylation processes in the spatio-temporal regulation of the priming and activation of enhancers has not yet been characterized. In this context, the aim of this study was to define the role of cytosine modifications (5mC and 5hmC) during the activation of enhancers bound by PFs. For this, we analyzed the implication of cytosine methylation and demethylation processes on enhancer priming and activation by using DNA methyltransferases or TET inhibitors. In addition, we identified the dynamics of enhancer priming and activation genome-wide during neural differentiation, in relation to the presence of 5hmC. The results allow us to propose a scheme of enhancer activation in which DNA methylation/demethylation dynamics play an essential role in the chromatin structure of these regulatory elements.

Epigenetic modifications, transcription factor (TF) availability and chromatin conformation influence how a genome is interpreted by the transcriptional machinery responsible for gene expression. Enhancers buried in non-coding regions are associated with significant differences in histone marks between different cell types. In contrast, gene promoters show more uniform modifications across cell types. In this report, enhancer identification is first carried out using an enhancer associated feature in mouse erythroid cells. Taking advantage of public domain ChIP-Seq data sets in mouse embryonic stem cells, an integrative model is then used to assess features in enhancer prediction, and subsequently locate enhancers. Significant associations with multiple TF bound loci, higher expression in the closest genes, and active enhancer marks support functionality and tissue-specificity of these enhancers. Motif enrichment analysis further determines known and novel TFs regulating the target cell type. Furthermore, the features identified can facilitate more accurate enhancer prediction in other cell types.

Epigenetic modifications, transcription factor (TF) availability and chromatin conformation influence how a genome is interpreted by the transcriptional machinery responsible for gene expression. Enhancers buried in non-coding regions are associated with significant differences in histone marks between different cell types. In contrast, gene promoters show more uniform modifications across cell types. In this report, enhancer identification is first carried out using an enhancer associated feature in mouse erythroid cells. Taking advantage of public domain ChIP-Seq data sets in mouse embryonic stem cells, an integrative model is then used to assess features in enhancer prediction, and subsequently locate enhancers. Significant associations with multiple TF bound loci, higher expression in the closest genes, and active enhancer marks support functionality and tissue-specificity of these enhancers. Motif enrichment analysis further determines known and novel TFs regulating the target cell type. Furthermore, the features identified can facilitate more accurate enhancer prediction in other cell types.

We previously described BT2, a BTB/POZ domain containing protein, as an activator of telomerase in Arabidopsis thaliana. In the current study, I present evidence of its interesting roles in mediating multiple hormone, stress and metabolic responses in plants. Steady-state expression of BT2 mRNA was regulated diurnally and was under the control of circadian clock, with a maximum expression in the dark. BT2 mRNA was responsive to nutrient status and to multiple biotic and abiotic stress signals. Using bt2 loss-of-function and BT2 over-expressing lines, I show that BT2 suppresses sugar and ABA-mediated responses during germination. BT2 is also essential for transcriptional gene activation mediated by CaMV 35S enhancers in Arabidopsis. Loss of BT2 in several well-characterized 35S enhancer activation-tagged lines such as yucca1d, pap1d, jaw1d etc., resulted in suppression of the activation phenotypes. The suppression of the phenotypes was due to decreased transcription of the activation-tagged genes. I further demonstrate that BT2 genetically interacts with CULLIN3. I propose that BT2 and CULLIN3 are components of a ubiquitin ligase complex. Together with associated proteins BET9 and BET10, the BT2 complex is required for CaMV 35S enhancer-mediated activation of gene expression and may regulate expression of target genes involved in multiple responses to fluctuating biotic and abiotic conditions. I also found that BT2 protein levels are tightly regulated in plants. BT2 protein was primarily localized in the nucleus and was developmentally regulated. BT2 turn-over was regulated in part by the 26S-proteosome, and rare codons present in its open reading frame affected BT2 protein accumulation. In addition to BT2, its orthologs, BT1, BT3, BT4 and BT5, also responded to light, clock and nutrients, with some differences. Moreover, BT1, BT3 and BT4 were also required for 35S enhancer-mediated activation of gene expression. I propose that BT family proteins assemble into multi-protein complexes to mediate multiple responses to changing environmental and nutritional conditions.

Gene expression is critical for animal development as cells divide and differentiate into multiple cell types. Cell-type specific gene expression is controlled by enhancers, DNA sequences that can direct expression of a target gene from hundreds of kilobases away. Gene promoters contact at least two enhancers on average, and enhancers may also contact each other. A key question is therefore how enhancers operate in this complex regulatory DNA context. It has long been assumed that enhancers act as independent modules based on their ability to drive gene expression when isolated in reporter constructs. To test assumptions of enhancer modularity, I probed interactions between two developmental enhancers from the even-skipped locus in Drosophila melanogaster blastoderm embryos. My results contradict the classic definition of enhancers; I found that the arrangement of enhancers relative to one another and the promoter influences levels of gene expression while not affecting its spatial pattern within the embryo. These results are described in Chapter 2. However, these enhancers are modular in one aspect: when fused directly together, they still direct their distinct spatial expression patterns. In Chapter 3 I describe a collaboration with Md Abul Hassan Samee in Saurabh Sinha's group at the University of Illinois Urbana-Champaign to apply computational sequence-to-expression models to my data. We found that a mechanistic model describing interactions between transcription factors was unable to fit our data well; in contrast, a phenomenological model that finds active sequences fits the data much better. These results indicate that to predict gene expression from sequence we will need to learn how enhancer boundaries are defined. In summary, I present evidence that the organization of enhancers within a locus impacts expression of the target gene. This finding overturns assumptions about enhancer modularity and emphasizes the importance of considering higher level interactions across a locus. Structural variation is common in natural populations, and our results highlight a novel way in which these sequence variants may alter gene expression. To realize the long-standing goal to predict gene expression directly from sequence we must investigate how enhancers interact within a complex locus.

The skin is an amazingly resilient and relatively impermeable barrier that provides protective, perceptive and communication functions to the body (Ramachandran & Fleisher, 2000). The stratum corneum is widely accepted as the barrier of the skin - limiting the transport of molecules into and across the skin. One of the bottlenecks in the successful development of transdermal drug delivery devices is the fact that the skin (more accurately, the stratum corneum - SC) tends to control the rate of drug transport. This makes it very difficult to influence or regulate the transdermal drug absorption kinetics from outside, Le. by means of the vehicle. A possible, and even elegant, solution may be the use of so-called "penetration enhancers", thereby suppressing the dominant role of the stratum corneum penetration barrier (Bodde et al., 1990). For this study 5-fluorouracil (5-FU), a polar hydrophilic drug, was chosen as model drug to study its penetration through the stratum corneum. Terpenes used as possible penetration enhancers for 5-FU were menthol, isomenthol, menthone, l3-myrcene, limonene and 1,8-cineole. In previous studies, terpenes with low skin irritancy and low systemic toxicity, were found to be effective penetration enhancers for a number of hydrophilic and lipophillic drugs (Cornwell & Barry, 1994; Cornwell et a/., 1996; Godwin & Michniak, 1999). The objective of this study was to determine the different flux rates of 5-FU in the absence of any pre-treatment of the stratum corneum and also through ethanol and selected terpene pre-treated SC. The effect of each terpene on the penetration of 5-FU was determined. The penetration of the selected terpenes themselves through the human stratum corneum was also determined in vitro permeation studies were performed using vertical Franz diffusion cells with human skin (stratum corneum). A saturated aqueous solution of 5-fluorouracil in the absence and presence of pre-treatment of the SC was used as the donor phase. Pre-treatment was performed by applying a 5 % terpene solution or absolute ethanol to the SC half an hour before the saturated III solution was applied in the donor compartment. A 50/50 ethanol/water solution was used as the receptor phase. All the experiments were conducted over a 24 h period. The 37°C temperature was held constant by means of a water bath. For the analysis of 5-FU flux rates, samples from the receptor compartment were obtained and were analysed by means of high-pressure liquid chromatography (HPLC). In order to determine the cumulative percentage of terpenes penetrated through human stratum corneum, the samples were analysed by gas chromatography (GC). In this study, only menthol and isomenthol (both oxygen-containing terpenes) showed a statistically significant increase on the flux of 5-FU, with flux values of 1.13 +- 0.38 and 1.45 +- 0.68 ug/cm2/h, respectively, compared to untreated skin with a flux value of 0.54 +- 0.23 ug/cm2/h for 5-FU. It was also proved that ethanol itself had an enhancing effect on 5-FU and showed synergistic effects on the enhancement activities of all the terpenes. It was found that all the terpenes (applied as a 5 % solution in ethanol) penetrated through the stratum corneum in the absence of 5-fluorouracil. 5-Fluorouracil had either an increasing or decreasing effect on the penetration of the terpenes. From these findings, it could be concluded that oxygen-containing terpenes had the best penetration enhancing effect on 5-FU and that menthol and isomenthol were the most effective penetration enhancers, although the extend of penetration enhancement is not large enough for clinical application. All the terpenes have the ability to penetrate through human stratum corneum, and 5-FU either had an increasing or decreasing effect on their penetration. / Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.

Acyclovir is an antiviral drug used in the treatment and prevention of herpes simplex and varicella-zoster viral infections. The major problem in the transdermal delivery of acyclovir is the permeation in sufficient amounts to deeper layers of the skin and into the systemic circulation. Acyclovir is a hydrophilic substance with a low partition coefficient, resulting in poor penetration through the excellent barrier of the skin, the stratum corneum. In an attempt to enhance the transdermal permeability of acyclovir, the aim of this study was to employ terpenes as possible penetration enhancers. Terpenes are constituents of natural essential oils, with widespread medicinal use including in aromatherapy. The terpenes used in this study were 1,8-cineole, limonene, menthol, menthone, and 13- myrcene. Terpenes are not only used as penetration enhancers, but are even more often present in drugs and cosmetics. Limited studies have been done concerning the penetration of terpenes through the skin. Thus, not only the effect of the terpenes on the penetration of acyclovir, but also the penetration of the terpenes themselves were studied. The influence of acyclovir on the penetration of the terpenes was also determined. In vitro permeation experiments were performed on human skin using Franz diffusion cells. The skin was pretreated with a 5 % solution of the terpene in ethanol and left for 30 minutes to enable ethanol evaporation and terpene incorporation into the skin. Saturated aqueous solutions of acyclovir (pH 7.4) were added in the donor compartment before and after skin pre-treatment. The acyclovir concentration retrieved from the receptor compartment of the Franz cells was analyzed by HPLC. The amount of terpene that penetrated were semi-quantitatively determined by GC. Penetration of acyclovir was significantly enhanced by two terpenes, viz. 1,8-cineole and menthol. The extent of enhancement was, however, not large enough to be of clinical use. The enhancement in acyclovir penetration observed upon ethanol pre-treatment alone, or in the presence of limonene, menthone or ~-myrcene, was not significant. Penetration enhancement of acyclovir by the terpenes was in accordance with previous studies, which postulated better enhancement of hydrophilic drugs by hydrophilic terpenes. Large percentages of the terpenes with log P values within the optimum log P range (1 - 3) penetrated, as was found with menthone and menthol. Penetration decreased accordingly as the log P, and thus lipophilicity, increased. Stratum corneum retention is regarded as the most plausible explanation for this phenomenon. In the case of 1,8- cineole, enhancer pooling in the stratum corneum could be a possible reason for its poor penetration. Acyclovir significantly influenced the penetration profiles of some of the terpenes, but no clear explanation could be given. / Thesis (M.Sc. (Pharm.))--North-West University, Potchefstroom Campus, 2004.

The skin is an amazingly resilient and relatively impermeable barrier that provides protective, perceptive and communication functions to the body (Ramachandran & Fleisher, 2000). The stratum corneum is widely accepted as the barrier of the skin - limiting the transport of molecules into and across the skin. One of the bottlenecks in the successful development of transdermal drug delivery devices is the fact that the skin (more accurately, the stratum corneum - SC) tends to control the rate of drug transport. This makes it very difficult to influence or regulate the transdermal drug absorption kinetics from outside, Le. by means of the vehicle. A possible, and even elegant, solution may be the use of so-called "penetration enhancers", thereby suppressing the dominant role of the stratum corneum penetration barrier (Bodde et al., 1990). For this study 5-fluorouracil (5-FU), a polar hydrophilic drug, was chosen as model drug to study its penetration through the stratum corneum. Terpenes used as possible penetration enhancers for 5-FU were menthol, isomenthol, menthone, l3-myrcene, limonene and 1,8-cineole. In previous studies, terpenes with low skin irritancy and low systemic toxicity, were found to be effective penetration enhancers for a number of hydrophilic and lipophillic drugs (Cornwell & Barry, 1994; Cornwell et a/., 1996; Godwin & Michniak, 1999). The objective of this study was to determine the different flux rates of 5-FU in the absence of any pre-treatment of the stratum corneum and also through ethanol and selected terpene pre-treated SC. The effect of each terpene on the penetration of 5-FU was determined. The penetration of the selected terpenes themselves through the human stratum corneum was also determined in vitro permeation studies were performed using vertical Franz diffusion cells with human skin (stratum corneum). A saturated aqueous solution of 5-fluorouracil in the absence and presence of pre-treatment of the SC was used as the donor phase. Pre-treatment was performed by applying a 5 % terpene solution or absolute ethanol to the SC half an hour before the saturated III solution was applied in the donor compartment. A 50/50 ethanol/water solution was used as the receptor phase. All the experiments were conducted over a 24 h period. The 37°C temperature was held constant by means of a water bath. For the analysis of 5-FU flux rates, samples from the receptor compartment were obtained and were analysed by means of high-pressure liquid chromatography (HPLC). In order to determine the cumulative percentage of terpenes penetrated through human stratum corneum, the samples were analysed by gas chromatography (GC). In this study, only menthol and isomenthol (both oxygen-containing terpenes) showed a statistically significant increase on the flux of 5-FU, with flux values of 1.13 +- 0.38 and 1.45 +- 0.68 ug/cm2/h, respectively, compared to untreated skin with a flux value of 0.54 +- 0.23 ug/cm2/h for 5-FU. It was also proved that ethanol itself had an enhancing effect on 5-FU and showed synergistic effects on the enhancement activities of all the terpenes. It was found that all the terpenes (applied as a 5 % solution in ethanol) penetrated through the stratum corneum in the absence of 5-fluorouracil. 5-Fluorouracil had either an increasing or decreasing effect on the penetration of the terpenes. From these findings, it could be concluded that oxygen-containing terpenes had the best penetration enhancing effect on 5-FU and that menthol and isomenthol were the most effective penetration enhancers, although the extend of penetration enhancement is not large enough for clinical application. All the terpenes have the ability to penetrate through human stratum corneum, and 5-FU either had an increasing or decreasing effect on their penetration. / Thesis (M.Sc.)--North-West University, Potchefstroom Campus, 2004.

Acyclovir is an antiviral drug used in the treatment and prevention of herpes simplex and varicella-zoster viral infections. The major problem in the transdermal delivery of acyclovir is the permeation in sufficient amounts to deeper layers of the skin and into the systemic circulation. Acyclovir is a hydrophilic substance with a low partition coefficient, resulting in poor penetration through the excellent barrier of the skin, the stratum corneum. In an attempt to enhance the transdermal permeability of acyclovir, the aim of this study was to employ terpenes as possible penetration enhancers. Terpenes are constituents of natural essential oils, with widespread medicinal use including in aromatherapy. The terpenes used in this study were 1,8-cineole, limonene, menthol, menthone, and 13- myrcene. Terpenes are not only used as penetration enhancers, but are even more often present in drugs and cosmetics. Limited studies have been done concerning the penetration of terpenes through the skin. Thus, not only the effect of the terpenes on the penetration of acyclovir, but also the penetration of the terpenes themselves were studied. The influence of acyclovir on the penetration of the terpenes was also determined. In vitro permeation experiments were performed on human skin using Franz diffusion cells. The skin was pretreated with a 5 % solution of the terpene in ethanol and left for 30 minutes to enable ethanol evaporation and terpene incorporation into the skin. Saturated aqueous solutions of acyclovir (pH 7.4) were added in the donor compartment before and after skin pre-treatment. The acyclovir concentration retrieved from the receptor compartment of the Franz cells was analyzed by HPLC. The amount of terpene that penetrated were semi-quantitatively determined by GC. Penetration of acyclovir was significantly enhanced by two terpenes, viz. 1,8-cineole and menthol. The extent of enhancement was, however, not large enough to be of clinical use. The enhancement in acyclovir penetration observed upon ethanol pre-treatment alone, or in the presence of limonene, menthone or ~-myrcene, was not significant. Penetration enhancement of acyclovir by the terpenes was in accordance with previous studies, which postulated better enhancement of hydrophilic drugs by hydrophilic terpenes. Large percentages of the terpenes with log P values within the optimum log P range (1 - 3) penetrated, as was found with menthone and menthol. Penetration decreased accordingly as the log P, and thus lipophilicity, increased. Stratum corneum retention is regarded as the most plausible explanation for this phenomenon. In the case of 1,8- cineole, enhancer pooling in the stratum corneum could be a possible reason for its poor penetration. Acyclovir significantly influenced the penetration profiles of some of the terpenes, but no clear explanation could be given. / Thesis (M.Sc. (Pharm.))--North-West University, Potchefstroom Campus, 2004.