Guidance for pediatric use in prescription information for novel medicinal products in the EU and the US.

Pediatric legislations in the European Union (EU) and the United States (US) have increased medicines approved for use in the pediatric population. Despite many similarities between these frameworks, the EU Paediatric Regulation more often provides regulators with a mandate to require pediatric drug development for novel medicinal products compared to US regulators. If used, this could give rise to differences in the guidance for pediatric use provided for clinicians in the two regions. However, the level of discordance in the guidance for pediatric use between the two regions is unknown. This cross-sectional study compares guidance for pediatric use in the EU Summary of Product Characteristics (SmPC) and the US Prescription Information (USPI) on the level of indications granted for novel medicinal products approved after the pediatric legislations came in to force in both regions. For all indications granted as of March 2020 for novel medicinal products approved in both regions between 2010 and 2018, we compared the guidance for pediatric use in the EU SmPC and the USPI. The guidance for pediatric use differed for 18% (61/348) of the listed indications covering 21% (45/217) of the products, but without the guidance being contradictory. Where guidance differed, an equal share was observed for indications with a higher level of information for pediatric use in one region over the other (49% (30/61) in the US; 51% (31/61) in the EU). The discrepancies in pediatric information could be explained by differences in regulations for 21% (13/61) of the indications. Only a few conditions and diseases (EU n = 4; US n = 1) were observed to cover potential pediatric use outside the approved adult indication. Although the EU Paediatric Regulation more often provides regulators a mandate for requiring pediatric drug development as compared to the US PREA, this was not reflected in the prescription information approved by the two regulatory authorities.

Ionic strength-dependent denaturation of Thermomyces lanuginosus lipase induced by SDS.

Triglyceride lipase from Thermomyces lanuginosus (TlL) has been reported to be resistant to denaturation by sodium dodecyl sulfate (SDS). We have found that at neutral pH, structural integrity is strongly dependent on ionic strength. In 10mM phosphate buffer and SDS, the lipase exhibits a far-UV CD spectrum similar to other proteins denatured in this surfactant while the near-UV CD spectrum shows a complete loss of tertiary structure, observations supported by steady state fluorescence spectroscopy. However, when increasing the ionic strength by the addition of NaCl, the lipase was rendered resistant towards SDS denaturation, as observed by all techniques employed. The effect of salt on the critical micelle concentration (CMC) of SDS was observed to correlate with the effect on the degree of SDS-induced denaturation. This finding is compatible with the notion that the concentration of SDS monomers is a crucial factor for SDS-lipase interactions. The presented results are important for the understanding and improvement of protein stability in surfactant systems.

Influence of acylation on the adsorption of insulin to hydrophobic surfaces.

PURPOSE: To study the effect of acylation on the adsorption of insulin to hydrophobic polystyrene beads. METHODS: Adsorption isotherms for adsorption of insulin and acylated insulin to hydrophobic polystyrene beads were established, and the adsorption of the two proteins was compared further with isothermal titration calorimetry. In addition, the secondary structure and the association behavior of the two proteins were studied with circular dichroism. RESULTS: Insulin and acylated insulin adsorbed with high affinity to the hydrophobic polystyrene beads. More acylated insulin molecules than insulin molecules adsorbed per unit surface area from solutions containing monomer-dimer mixtures of acylated insulin and insulin, respectively. In contrast, no difference was observed in the number of insulin and acylated insulin molecules adsorbing per unit surface area, when adsorption occurred from solutions containing monomer-dimer-hexamer mixtures. CONCLUSION: The influence of acylation on the adsorption behavior of insulin depends on the association degree of insulin, possibly due to a greater difference in hydrophobicity between monomeric insulin and acylated insulin than between the hexameric forms of these two proteins.

A helical structural nucleus is the primary elongating unit of insulin amyloid fibrils.

Although amyloid fibrillation is generally believed to be a nucleation-dependent process, the nuclei are largely structurally uncharacterized. This is in part due to the inherent experimental challenge associated with structural descriptions of individual components in a dynamic multi-component equilibrium. There are indications that oligomeric aggregated precursors of fibrillation, and not mature fibrils, are the main cause of cytotoxicity in amyloid disease. This further emphasizes the importance of characterizing early fibrillation events. Here we present a kinetic x-ray solution scattering study of insulin fibrillation, revealing three major components: insulin monomers, mature fibrils, and an oligomeric species. Low-resolution three-dimensional structures are determined for the fibril repeating unit and for the oligomer, the latter being a helical unit composed of five to six insulin monomers. This helical oligomer is likely to be a structural nucleus, which accumulates above the supercritical concentration used in our experiments. The growth rate of the fibrils is proportional to the amount of the helical oligomer present in solution, suggesting that these oligomers elongate the fibrils. Hence, the structural nucleus and elongating unit in insulin amyloid fibrillation may be the same structural component above supercritical concentrations. A novel elongation pathway of insulin amyloid fibrils is proposed, based on the shape and size of the fibrillation precursor. The distinct helical oligomer described in this study defines a conceptually new basis of structure-based drug design against amyloid diseases.

Data-enriched edible pharmaceuticals (DEEP) of medical cannabis by inkjet printing.

Medical cannabis has shown to be effective in various diseases that have not successfully been treated with other marketed drug products. However, the dose of cannabis is highly individual and additionally, medical cannabis is prone to misuse. To combat these challenges, the concept of data-enriched edible pharmaceuticals (DEEP) is introduced. Quick Response (QR) code patterns containing lipophilic cannabinoids, i.e., cannabidiol (CBD) and delta-9-tetrahydrocannabinol (THC), were printed using a desktop inkjet printer. This allows for simultaneously printing an individual dose and encapsulating information relevant to the end-users and other stakeholders in a single dosage unit, which is readable by a standard smartphone. Different doses of CBD and THC were incorporated in the DEEP by printing various (1-10) layers of the cannabinoid-containing ink on porous substrates, i.e., solid foams, prepared by solvent casting and subsequent freeze-drying. The printed DEEP were still readable after 8 weeks of storage in dry and cold conditions. This approach of 'in-drug labeling' instead of 'drug package labeling' provides a new possibility for developing a more efficient supply chain of pharmaceuticals and safer medication schemes by increasing the traceability of drug products at a single dosage unit level.

Cellular uptake and membrane-destabilising properties of alpha-peptide/beta-peptoid chimeras: lessons for the design of new cell-penetrating peptides.

Novel peptidomimetic backbone designs with stability towards proteases are of interest for several pharmaceutical applications including intracellular delivery. The present study concerns the cellular uptake and membrane-destabilising effects of various cationic chimeras comprised of alternating N-alkylated beta-alanine and alpha-amino acid residues. For comparison, homomeric peptides displaying octacationic functionalities as well as the Tat(47-57) sequence were included as reference compounds. Cellular uptake studies with fluorescently labelled compounds showed that guanidinylated chimeras were taken up four times more efficiently than Tat(47-57). After internalisation, the chimeras were localised primarily in vesicular compartments and diffusively in the cytoplasm. In murine NIH3T3 fibroblasts, the chimeras showed immediate plasma membrane permeabilising properties, which proved highly dependent on the chimera chain length, and were remarkably different from the effects induced by Tat(47-57). Finally, biophysical studies on model membranes showed that the chimeras in general increase the permeability of fluid phase and gel phase phosphatidylcholine (PC) vesicles without affecting membrane acyl chain packing, which suggests that they restrict lateral diffusion of the membrane lipids by interaction with phospholipid head groups. The alpha-peptide/beta-peptoid chimeras described herein exhibit promising cellular uptake properties, and thus represent proteolytically stable alternatives to currently known cell-penetrating peptides.

Danish Physicians' Views on the Appropriateness of the Involvement of Patients with Type 2 Diabetes in Regulatory Decision Making: A Qualitative Study.

BACKGROUND: Regulators, the pharmaceutical industry, and patient organizations expect an increased inclusion of patients' risk preferences in medical regulatory decisions, for example, with regard to market approval. Merging of input from patients with, for example, multiple sclerosis, with expertise from health professionals in regulatory decisions has already occurred. The complex task of involving larger and more heterogeneous patient populations (e.g. with diabetes mellitus, asthma), however, remains. OBJECTIVE: This study aimed to understand physicians' experiences with factors influencing patients with diabetes mellitus perceived risks of their medicines and to explore how physicians, based on these experiences, perceive patients with diabetes to be suited for involvement in regulatory decisions. This study will provide knowledge that can improve the inclusion of heterogeneous patient groups in regulatory decisions. METHODS: We conducted five semi-structured interviews with physicians with different types of experiences with patients' risk perceptions (for example, being in contact with individual patients vs. being involved in developing guidelines at the population level) and one focus group interview with eight general practitioners in Sjælland, Denmark. We applied a thematic analysis to explore physicians' experiences of the risk perceptions of patients with type 2 diabetes and their perceptions of patients' fitness for involvement in regulatory decisions. RESULTS: The risk perceptions and preferences of patients with diabetes were perceived to be rather diverse. Four drivers behind this diversity were described: past experiences, personality, prognosis ability, and knowledge. The legitimacy of patient preferences was not questioned, but the diversity of risk perceptions made the respondents question the existence of a uniform 'patient voice' useful for regulatory decision making. CONCLUSION: The respondents acknowledged the relevance and legitimacy of the patient perspective, but it was a concern that patient risk perceptions, at present, are too diverse to be included in regulatory decisions. Whether patients make regulatory decisions as perceived by physicians needs to be confirmed by future studies.

Cloning of the pig PEPT2 (pPEPT2) and characterization of the effects of epidermal growth factor (EGF) on pPEPT2-mediated peptide uptake in the renal porcine cell line LLC-PK1.

The renal di/tri-peptide transporter PEPT2 is situated in the distal parts of the proximal tubule, where it mediates reabsorption of peptides from the primary urine. The transporter has been thoroughly characterised with respect to substrate-affinity relationships, however little is known about its regulation. Previous studies from our group have shown that epidermal growth factor (EGF) down-regulates PepT2 in the rat proximal kidney tubule cell line SKPT0193 cl.2. The aim of the present work was to clone the pig PEPT2 (pPEPT2) and to study the effect of EGF on pPEPT2 expression in the porcine kidney cell line LLC-PK1. pPEPT2 from LLC-PK1 cells was PCR-cloned. The predicted protein consisted of 729 amino acids, had a molecular mass of 81.7 kDa and was 88% identical and 94% similar to hPEPT2, thus displaying a close similarity to the human orthologue. pPepT2 expressing LLC-PK1 cells were cultured in the absence and presence of EGF in the culture media. EGF induced an increase in uptake of (14)C-glycylsarcosine ([(14)C]-Gly-Sar), accompanied by an increase in transcellular electrical resistance, total cell protein, alkaline phosphatase activity and cell density. The increase in uptake of [(14)C]-Gly-Sar was maximal when cells were cultured in the presence of EGF throughout the culture period of 10 days. The EGF-treatment did not induce significant changes in pPepT2 mRNA expression, as determined by real-time PCR. The effect of EGF thus appears to be an increase in the number of cells without a loss of differentiation, an effect which is quite different from earlier observations on the SKPT cell line.

Protein drug stability: a formulation challenge.

The increasing use of recombinantly expressed therapeutic proteins in the pharmaceutical industry has highlighted issues such as their stability during long-term storage and means of efficacious delivery that avoid adverse immunogenic side effects. Controlled chemical modifications, such as substitutions, acylation and PEGylation, have fulfilled some but not all of their promises, while hydrogels and lipid-based formulations could well be developed into generic delivery systems. Strategies to curb the aggregation and misfolding of proteins during storage are likely to benefit from the recent surge of interest in protein fibrillation. This might in turn lead to generally accepted guidelines and tests to avoid unforeseen adverse effects in drug delivery.

Liposomes for phospholipase A2 triggered siRNA release: preparation and in vitro test.

Small interfering RNA (siRNA) is potent and highly specific for gene silencing. However, for therapeutic applications, delivery systems are required to protect siRNA from degradation, to enhance cellular uptake and for site-specific delivery. We used a double emulsion technique to encapsulate siRNA into stealth liposomes (SL) to increase entrapment efficiency compared to passive encapsulation. SL are designed for localized, active release of siRNA by secretory phosholipase A(2) (sPLA(2)). sPLA(2) acts as a site-specific enzymatic trigger that actively degrades the liposomal carrier in inflamed tissue releasing entrapped drug. Relatively good encapsulation efficiencies compared to passive encapsulation were demonstrated (7-9%) and SL size was appropriate for i.v. administration (60-90 nm). siRNA targeting enhanced green fluorescent protein (EGFP) entrapped in SL did not silence gene expression of HeLa-cells stably expressing EGFP. However, preliminary flow cytometry and confocal microscopy data showed that the SL siRNA formulation increased uptake of siRNA into vesicular compartments of HeLa-cells in a concentration-dependent manner that could be augmented by exogenuos sPLA(2). We hypothesize that the SL can be used to target siRNA to inflammed tissue for silencing of cytokine expression in rheumatoid arthritis.

125I used for labelling of proteins in an absorption model changes the absorption rate of insulin aspart.

UNLABELLED: The aim of this study is to validate the ability of the disappearance model to predict absorption rates of insulin aspart in pigs. The disappearance model is used as a screening tool to estimate absorption rates after subcutaneous injections in humans or pigs especially of insulin and insulin analogues. The disappearance model measures remaining radioactivity at the injection site and therefore radioactive labelling of the insulin analogue is necessary. The labelling is done with 125I. One of the assumptions for the disappearance model to be reliable is that absorption rates of the labelled and non-labelled molecules are comparable. In this study, we compared disappearance data with absorption calculated from plasma samples of insulin aspart. The calculated absorption is based on non-labelled insulin aspart. The absorption rate from the disappearance data was statistical significant (p = 0.0028) different from the absorption rate based on plasma samples. A control study was carried out where 125I labelled insulin aspart was compared to 127I (the natural non-radioactive isotope) insulin aspart. In this study, absorption rate from the disappearance data and absorption rate based on plasma samples were similar (p = 0.63). CONCLUSION: Iodination of insulin aspart changes the subcutaneous absorption rate.

Computational prediction of solubilizers' effect on partitioning.

A computational model for the prediction of solubilizers' effect on drug partitioning has been developed. Membrane/water partitioning was evaluated by means of immobilized artificial membrane (IAM) chromatography. Four solubilizers were used to alter the partitioning in the IAM column. Two types of molecular descriptors were calculated: 2D descriptors using the MOE software and 3D descriptors using the Volsurf software. Structure-property relationships between each of the two types of descriptors and partitioning were established using partial least squares, projection to latent structures (PLS) statistics. Statistically significant relationships between the molecular descriptors and the IAM data were identified. Based on the 2D descriptors structure-property relationships R(2)Y=0. 99 and Q(2)=0.82-0.83 were obtained for some of the solubilizers. The most important descriptor was related to logP. For the Volsurf 3D descriptors models with R(2)Y=0.53-0.64 and Q(2)=0.40-0.54 were obtained using five descriptors. The present study showed that it is possible to predict partitioning of substances in an artificial phospholipid membrane, with or without the use of solubilizers.

Characterisation and physical stability of PEGylated glucagon.

Glucagon was mono-PEGylated with PEG 5000 at Lys-12 to examine the effect on conformation and physical stability during purification and freeze-drying. The model peptide glucagon is highly unstable and readily forms fibrils in solution. Secondary structure was determined by FTIR and far-UV CD and physical stability was assessed by the Thioflavin T assay. Glucagon samples were included, which underwent the same RP-HPLC purification and/or freeze-drying as glucagon-PEG 5000. After purification and freeze-drying glucagon samples showed formation of intermolecular beta-sheet by FTIR, this correlated with shorter lag-times for fibrillation in the Thioflavin T assay. Formation of intermolecular beta-sheet was less apparent for glucagon-PEG 5000 and no fibrillation was detected by Thioflavin T assay. Apparently PEGylation significantly improved the physical stability of glucagon after purification and freeze-drying, possibly by steric hindrance of peptide-peptide interactions. Alterations in the secondary structure were observed for freeze-dried and reconstituted peptide samples by liquid FTIR. The peak for alpha-helix shifted to 1664 cm(-1), which could possibly be explained by formation of 3(10)-helix. Neither 3(10)-helix nor intermolecular beta-sheet could be detected by far-UV CD, where all peptide samples showed similar spectra. In conclusion, glucagon-PEG 5000 showed a significantly improved physical stability during purification and freeze-drying compared to glucagon.

Risk Perceptions in Diabetic Patients Who Have Experienced Adverse Events: Implications for Patient Involvement in Regulatory Decisions.

BACKGROUND: Increasingly, patients are expected to influence decisions previously reserved for regulatory agencies, pharmaceutical companies, and healthcare professionals. Individual patients have previously represented their patient population when rare, serious adverse events (AEs) were weighed as part of a benefit-risk assessment. However, the degree of heterogeneity of the patient population is critical for how accurately they can be represented by individuals. OBJECTIVES: This study aims to explore patients' risk perception of rare, serious adverse effects of medicines with regard to blood glucose-lowering antidiabetics used by the individual patient. METHODS: Semi-structured interviews were conducted with 18 patients with diabetes with self-perceived serious, but not necessarily rare, AEs (e.g. stroke or valve or bypass surgery). The interviews explored the patients' history of disease, perceptions of the terms rare and serious, and overall levels of risk aversion. A thematic analysis of the interviews, including a consensus discussion, was carried out. RESULTS: Interestingly, respondents rarely made a clear distinction between medicines-induced AEs and complications related to disease progression. Concerns regarding AEs were apparently diverse but were systematically related to the personal experiences of the respondents. Respondents routinely ignored information about possible rare, serious AEs, unless it could be related to personal experience. In the absence of experience, concerns were focused on common and less serious AEs, thus disregarding rare and more serious events. CONCLUSION: The study suggests that experience of AEs, related to either medicines or disease, constitutes an important factor of patient risk perception. We therefore propose that serious adverse experiences should be added to the traditional panel of socioeconomic factors that are accounted for when patients are invited to give input on regulatory decisions.

Interfacial adsorption of insulin conformational changes and reversibility of adsorption.

The adsorption of human insulin to Teflon particles was studied with respect to conformational changes and the reversibility of adsorption was examined by total internal reflection fluorescence (TIRF). Adsorption isotherms for the adsorption of human insulin indicated high affinity adsorption, even at electrostatic repulsive conditions. The plateau value for adsorption was in accordance with a protein layer consisting primarily of insulin monomers. Conformational changes of the insulin upon adsorption, was investigated by circular dicroism (CD) and fluorescence spectroscopy. The results suggested unfolding of adsorbed insulin, as observed by a decrease in alpha-helix and increase in random coil conformation. The changes in protein structure was not only related to the adsorbed species being monomeric, since CD and fluorescence results were different for adsorbed insulin compared to a monomeric analog of human insulin. Furthermore, the thermal stability in the adsorbed state was changed compared to insulin in solution. On the basis of the TIRF studies with FITC-labelled insulin it was not possible to firmly conclude whether exchange between human insulin in the adsorbed state and in solution takes place, due to the limited time range investigated. However, the desorption mechanism appeared to be different with unlabelled insulin in the bulk solution compared to phosphate buffer.

In vitro evaluation of N-methyl amide tripeptidomimetics as substrates for the human intestinal di-/tri-peptide transporter hPEPT1.

Oral absorption of tripeptides is generally mediated by the human intestinal di-/tri-peptide transporter, hPEPT1. However, the bioavailability of tripeptides is often limited due to degradation in the GI-tract by various peptidases. The aim of the present study was to evaluate the general application of N-methyl amide bioisosteres as peptide bond replacements in tripeptides in order to decrease degradation by peptidases and yet retain affinity for and transport via hPEPT1. Seven structurally diverse N-methyl amide tripeptidomimetics were selected based on a principal component analysis of structural properties of 6859 N-methyl amide tripeptidomimetics. In vitro extracellular degradation of the selected tripeptidomimetics as well as affinity for and transepithelial transport via hPEPT1 were investigated in Caco-2 cells. Decreased apparent degradation was observed for all tripeptidomimetics compared to the corresponding natural tripeptides. However, affinity for and transepithelial transport via hPEPT1 were only seen for Gly-Sar-Sar, AsnPsi[CONCH(3)]PhePsi[CONCH(3)]Trp, and Gly-Sar-Leu. This implies that tripeptidomimetics originating from tripeptides with neutral side chains are more likely to be substrates for hPEPT1 than tripeptidomimetics with charged side chains. The results of the present study indicate that the N-methyl amide peptide bond replacement approach for increasing bioavailability of tripeptidomimetic drug candidates is not generally applicable to all tripeptides. Nevertheless, retained affinity for and transport via hPEPT1 were shown for three of the evaluated N-methyl amide tripeptidomimetics.

Factors of importance for a successful delivery system for proteins.

Protein pharmaceuticals have matured into an important class of drugs, now comprising one in three novel drugs introduced on the market. However, significant gains are still to be made in reducing the costs of production, ensuring proper pharmacokinetics and efficacy, increasing patient compliance and convenience, and reducing side effects such as immunogenicity. This review summarises these issues and provides recent examples of methods to reduce costs, alter pharmacokinetics and increase patient compliance. It also discusses the increasing interest in understanding immunogenicity in order to prevent failure of the protein drug or serious life-threatening side effects due to autoimmunogenicity.

Delivery aspects of small peptides and substrates for peptide transporters.

The present summary highlight chemical strategies applied to improve plasma half-lives and oral bioavailability of peptidic drugs as well as view on intestinal and pancreatic peptidase mediated degradation of peptidic drugs. In general chemical strategies used to increase the oral bioavailability of peptidic drugs consisting of more than three amino acids is disappointing. On the other hand chemical approaches to stabilize peptidic drugs against metabolism seem promising for increasing plasma half-lives of parental peptidic drugs as well as for increasing oral bioavailability of di/tripeptidomimetics and dipeptidyl pro-moieties targeting peptide transporters.

Particle size and surface charge affect particle uptake by human dendritic cells in an in vitro model.

Current vaccine development includes optimization of antigen delivery to antigen presenting cells, such as dendritic cells (DC). Particulate systems have attracted increasing attention in the development of vaccine delivery systems. In the present study, we investigated DC uptake of model fluorescent polystyrene particles with a broad size range and variable surface properties. Localization of particles was investigated using confocal laser scanning microscopy and uptake was quantified by flow cytometry. Immature DC were generated from mononuclear cells isolated from human blood. The polystyrene particles interacted with the DC throughout the tested diameter range of 0.04-15 microm in a time- and concentration-dependent manner. The optimal particle diameter for fast and efficient acquisition by a substantial percentage of the DC was 0.5 microm and below. The surface of 1 and 0.1 microm polystyrene particles was covalently modified with different polyaminoacids/proteins, yielding particles with varying surface charge. Uptake of 1 microm particles was greatly enhanced when particles displayed a positive surface charge. In general, the present findings establish that particle diameters of 0.5 microm and below were optimal for DC uptake; however uptake of larger particles could be greatly enhanced by rendering the particle surface positive. Whether increased particle uptake is correlated with increased immune responses, remains to be established.

Prodrugs of purine and pyrimidine analogues for the intestinal di/tri-peptide transporter PepT1: affinity for hPepT1 in Caco-2 cells, drug release in aqueous media and in vitro metabolism.

A general drug delivery approach for increasing oral bioavailability of purine and pyrimidine analogues such as acyclovir may be to link these compounds reversibly to stabilized dipeptide pro-moieties with affinity for the human intestinal di/tri-peptide transporter, hPepT1. In the present study, novel L-Glu-Sar and D-Glu-Ala ester prodrugs of acyclovir and 1-(2-hydroxyethyl)-linked thymine were synthesized and their affinities for hPepT1 in Caco-2 cells were determined. Furthermore, the degradation of the prodrugs was investigated in various aqueous and biological media and compared to the corresponding hydrolysis of the prodrug valaciclovir. Affinity studies showed that the L-Glu-Sar prodrugs had high affinity for hPepT1 (K(i) approximately 0.2-0.3 mM), whereas the D-Glu-Ala prodrugs had poor affinity (K(i) approximately 50 mM). The pH-rate profiles of the prodrugs D-Glu[1-(2-hydroxyethyl)thymine]-Ala and L-Glu[acyclovir]-Sar showed specific base catalyzed degradation at pH above 4.5 and 5.5, respectively. This implicates that the degradation rates at pH approximately 7.4 (t(1/2) approximately 3.5 and 5.5 h) are approximately 25 times faster than at upper small intestinal pH approximately 6.0. In 10% porcine intestinal homogenate and 80% human plasma the half-lives of the L-Glu-Sar prodrugs were approximately between 45 and 90 min indicating a limited enzyme catalyzed degradation. In contrast, valaciclovir underwent extensive enzyme catalyzed hydrolysis in 10% porcine intestinal homogenate (t(1/2) approximately 1 min). In conclusion, L-Glu-Sar may potentially function as pro-moiety for purine and pyrimidine analogues, where release of parent compound primarily is controlled by a specific base catalyzed hydrolysis. Acyclovir is quantitatively released at the relevant pH 7.4, whereas the 1-(2-hydroxyethyl)-linked thymine is released instead of the parent compound thymine.