Reversible Lipidization Prolongs the Pharmacological Effect, Plasma Duration, and Liver Retention of Octreotide

Derivatization with fatty acids in peptide and protein drug discovery

Peptides and proteins are widely used to treat a range of medical conditions; however, they often have to be injected and their effects are short-lived. These shortcomings of the native structure can be addressed by molecular engineering, but this is a complex undertaking. A molecular engineering technology initially applied to insulin - and which has now been successfully applied to several biopharmaceuticals - entails the derivatization of peptides and proteins with fatty acids. Various protraction mechanisms are enabled by the specific characteristics and positions of the attached fatty acid. Furthermore, the technology can ensure a long half-life following oral administration of peptide drugs, can alter the distribution of peptides and may hold potential for tissue targeting. Due to the inherent safety and well-defined chemical nature of the fatty acids, this technology provides a versatile approach to peptide and protein drug discovery.

Oromucosal delivery of macromolecules: Challenges and recent developments to improve bioavailability

Owing to their biological diversity, high potency, good tolerability, low immunogenicity, site-specific activity, and great efficacy, macromolecular drugs (i.e., proteins and peptides, antibodies, hormones, nucleic acids, vaccines, etc.) are extensively used as diagnostics, prophylactics, and therapeutics in various diseases. To overcome drawbacks associated with parenteral (invasive) delivery of macromolecules as well as to preserve their therapeutic integrity, oromucosal route (sublingual and buccal) has been proven efficient alternate port of delivery. This review aims to summarize challenges associated with oromucosal route and overtime developments in conventional delivery systems with special emphasis on most recent delivery strategies. Over the past few decades, significant efforts have been made for improving the oromucosal absorption of macromolecules by employing chemical penetration enhancers (CPE), enzyme inhibitors, chemical modification of drug structure (i.e., lipidation, PEGylation, etc.), and mucoadhesive materials in the form of buccal tablets, films (or patches), sprays, fast disintegrating tablets, and microneedles. Adaptation of adjunct strategies (e.g., iontophoresis in conjunction with CPE) has shown significant improvement in oromucosal absorption of macromolecules; however, these approaches were also associated with many drawbacks. To overcome these shortcomings and to further improve therapeutic outcomes, specialized delivery devices called "hybrid nanosystems" have been designed in recent times. This newer intervention showed promising potential for promoting oromucosal absorption and absolute bioavailability of macromolecules along with improved thermostability (cold chain free storage), enabling self-administration, site-specific activity, improving therapeutic efficacy and patient compliance. We anticipate that tailoring of hybrid nanosystems to clinical trials as well as establishing their short- and long-term safety profile would substantiate their therapeutic value as pharmaceutical devices for oromucosal delivery of macromolecules.

Characterization and impact of peptide physicochemical properties on oral and subcutaneous delivery

Peptides, an emerging modality within the biopharmaceutical industry, are often delivered subcutaneously with evolving prospects on oral delivery. Barrier biology within the subcutis or gastrointestinal tract is a significant challenge in limiting absorption or otherwise disrupting peptide disposition. Aspects of peptide pharmacokinetic performance and ADME can be mitigated with careful molecular design that tailors for properties such as effective size, hydrophobicity, net charge, proteolytic stability, and albumin binding. In this review, we endeavor to highlight effective techniques in qualifying physicochemical properties of peptides and discuss advancements of in vitro models of subcutaneous and oral delivery. Additionally, we will delineate empirical findings around the relationship of these physicochemical properties and in vivo (animal or human) impact. We conclude that robust peptide characterization methods and in vitro techniques with demonstrated correlations to in vivo data are key routines to incorporate in the drug discovery and development to improve the probability of technical and commercial success of peptide therapeutics.

Opportunities and challenges of fatty acid conjugated therapeutics

Instability, poor cellular uptake and unfavorable pharmacokinetics and biodistribution of many therapeutic molecules require modification in their physicochemical properties. The conjugation of these APIs with fatty acids has demonstrated an enhancement in their lipophilicity and stability. The improvement in the formulations that resulted from the conjugation of a drug with a fatty acid includes increased half-life, enhanced cellular uptake and retention, targeted tumor delivery, reduced chemoresistance in cancer, and improved blood-brain-barrier (BBB) penetration. In this review, various therapeutic molecules, including small molecules, peptides and oligonucleotides, that have been conjugated with fatty acid have been thoroughly discussed along with various conjugation strategies. The application of nano-system based delivery is gaining a lot of attention due to its ability to provide controlled drug release, targeting and reducing the extent of side effects. This review also covers various nano-carriers that have been utilized for the delivery of fatty acid drug conjugates. The enhanced lipophilicity of the drug-fatty acid conjugate has shown to enhance the affinity of the drug towards these carriers, thereby increasing the entrapment efficiency and formulation performance.

Position of lipidation influences anticancer activity of Smac analogs

A small group of lipid-conjugated Smac mimetics was synthesized to probe the influence of the position of lipidation on overall anti-cancer activity. Specifically, new compounds were modified with lipid(s) in position 3 and C-terminus. Previously described position 2 lipidated analog M11 was also synthesized. The resulting mini library of Smacs lipidated in positions 2, 3 and C-terminus was screened extensively in vitro against a total number of 50 diverse cancer cell lines revealing that both the position of lipidation as well as the type of lipid, influence their anti-cancer activity and cancer type specificity. Moreover, when used in combination therapy with inhibitor of menin-MLL1 protein interactions, position 2 modified analog SM2 showed strong synergistic anti-cancer properties. The most promising lipid-conjugated analogs SM2 and SM6, showed favorable pharmacokinetics and in vivo activity while administered subcutaneously in the preclinical mouse model. Collectively, our findings suggest that lipid modification of Smacs may be a viable approach in the development of anti-cancer therapeutic leads.

A review of lipidation in the development of advanced protein and peptide therapeutics

The use of biologics (peptide and protein based drugs) has increased significantly over the past few decades. However, their development has been limited by their short half-life, immunogenicity and low membrane permeability, restricting most therapies to extracellular targets and administration by injection. Lipidation is a clinically-proven post-translational modification that has shown great promise to address these issues: improving half-life, reducing immunogenicity and enabling intracellular uptake and delivery across epithelia. Despite its great potential, lipidation remains an underutilized strategy in the clinical translation of lead biologics. We review how lipidation can overcome common challenges in biologics development as well as highlight gaps in our understanding of the effect of lipidation on therapeutic efficacy, where increased research and development efforts may lead to next-generation drugs.

Peptide Lipidation - A Synthetic Strategy to Afford Peptide Based Therapeutics

Peptide and protein aberrant lipidation patterns are often involved in many diseases including cancer and neurological disorders. Peptide lipidation is also a promising strategy to improve pharmacokinetic and pharmacodynamic profiles of peptide-based drugs. Self-adjuvanting peptide-based vaccines commonly utilise the powerful TLR2 agonist Pam_nCys lipid to stimulate adjuvant activity. The chemical synthesis of lipidated peptides can be challenging hence efficient, flexible and straightforward synthetic routes to access homogeneous lipid-tagged peptides are in high demand. A new technique coined Cysteine Lipidation on a Peptide or Amino acid (CLipPA) uses a 'thiol-ene' reaction between a cysteine and a vinyl ester and offers great promise due to its simplicity, functional group compatibility and selectivity. Herein a brief review of various synthetic strategies to access lipidated peptides, focusing on synthetic methods to incorporate a Pam_nCys motif into peptides, is provided.

Lipid-conjugated Smac analogues

A small library of monovalent and bivalent Smac mimics was synthesized based on 2 types of monomers, with general structure NMeAla-Xaa-Pro-BHA (Xaa=Cys or Lys). Position 2 of the compounds was utilized to dimerize both types of monomers employing various bis-reactive linkers, as well as to modify selected compounds with lipids. The resulting library was screened in vitro against metastatic human breast cancer cell line MDA-MB-231, and the two most active compounds selected for in vivo studies. The most active lipid-conjugated analogue M11, showed in vivo activity while administered both subcutaneously and orally. Collectively, our findings suggest that lipidation may be a viable approach in the development of new Smac-based therapeutic leads.

Acylation of salmon calcitonin modulates in vitro intestinal peptide flux through membrane permeability enhancement

Acylation of peptide drugs with fatty acid chains has proven beneficial for prolonging systemic circulation, as well as increasing enzymatic stability and interactions with lipid cell membranes. Thus, acylation offers several potential benefits for oral delivery of therapeutic peptides, and we hypothesize that tailoring the acylation may be used to optimize intestinal translocation. This work aims to characterize acylated analogues of the therapeutic peptide salmon calcitonin (sCT), which lowers blood calcium, by systematically increasing acyl chain length at two positions, in order to elucidate its influence on intestinal cell translocation and membrane interaction. We find that acylation drastically increases in vitro intestinal peptide flux and confers a transient permeability enhancing effect on the cell layer. The analogues permeabilize model lipid membranes, indicating that the effect is due to a solubilization of the cell membrane, similar to transcellular oral permeation enhancers. The effect is dependent on pH, with larger effect at lower pH, and is impacted by acylation chain length and position. Compared to the unacylated peptide backbone, N-terminal acylation with a short chain provides 6- or 9-fold increase in peptide translocation at pH 7.4 and 5.5, respectively. Prolonging the chain length appears to hamper translocation, possibly due to self-association or aggregation, although the long chain acylated analogues remain superior to the unacylated peptide. For K(18)-acylation a short chain provides a moderate improvement, whereas medium and long chain analogues are highly efficient, with a 12-fold increase in permeability compared to the unacylated peptide backbone, on par with currently employed oral permeation enhancers. For K(18)-acylation the medium chain acylation appears to be optimal, as elongating the chain causes greater binding to the cell membrane but similar permeability, and we speculate that increasing the chain length further may decrease the permeability. In conclusion, acylated sCT acts as its own in vitro intestinal permeation enhancer, with reversible effects on Caco-2 cells, indicating that acylation of sCT may represent a promising tool to increase intestinal permeability without adding oral permeation enhancers.

A two-step strategy to enhance activity of low potency peptides

Novel strategies are needed to expedite the generation and optimization of peptide probes targeting G protein-coupled receptors (GPCRs). We have previously shown that membrane tethered ligands (MTLs), recombinant proteins comprised of a membrane anchor, an extracellular linker, and a peptide ligand can be used to identify targeted receptor modulators. Although MTLs provide a useful tool to identify and/or modify functionally active peptides, a major limitation of this strategy is the reliance on recombinant protein expression. We now report the generation and pharmacological characterization of prototype peptide-linker-lipid conjugates, synthetic membrane anchored ligands (SMALs), which are designed as mimics of corresponding MTLs. In this study, we systematically compare the activity of selected peptides as MTLs versus SMALs. As prototypes, we focused on the precursor proteins of mature Substance P (SubP) and Cholecystokinin 4 (CCK4), specifically non-amidated SubP (SubP-COOH) and glycine extended CCK4 (CCK4-Gly-COOH). As low affinity soluble peptides these ligands each presented a challenging test case for assessment of MTL/SMAL technology. For each ligand, MTLs and corresponding SMALs showed agonist activity and comparable subtype selectivity. In addition, our results illustrate that membrane anchoring increases ligand potency. Furthermore, both MTL and SMAL induced signaling can be blocked by specific non-peptide antagonists suggesting that the anchored constructs may be orthosteric agonists. In conclusion, MTLs offer a streamlined approach for identifying low activity peptides which can be readily converted to higher potency SMALs. The ability to recapitulate MTL activity with SMALs extends the utility of anchored peptides as probes of GPCR function.

Acylation of Glucagon-like peptide-2: interaction with lipid membranes and in vitro intestinal permeability

Background

Acylation of peptide drugs with fatty acid chains has proven beneficial for prolonging systemic circulation as well as increasing enzymatic stability without disrupting biological potency. Acylation has furthermore been shown to increase interactions with the lipid membranes of mammalian cells. The extent to which such interactions hinder or benefit delivery of acylated peptide drugs across cellular barriers such as the intestinal epithelia is currently unknown. The present study investigates the effect of acylating peptide drugs from a drug delivery perspective.

Purpose

We hypothesize that the membrane interaction is an important parameter for intestinal translocation, which may be used to optimize the acylation chain length for intestinal permeation. This work aims to characterize acylated analogues of the intestinotrophic Glucagon-like peptide-2 by systematically increasing acyl chain length, in order to elucidate its influence on membrane interaction and intestinal cell translocation in vitro.

Results

Peptide self-association and binding to both model lipid and cell membranes was found to increase gradually with acyl chain length, whereas translocation across Caco-2 cells depended non-linearly on chain length. Short and medium acyl chains increased translocation compared to the native peptide, but long chain acylation displayed no improvement in translocation. Co-administration of a paracellular absorption enhancer was found to increase translocation irrespective of acyl chain length, whereas a transcellular enhancer displayed increased synergy with the long chain acylation.

Conclusions

These results show that membrane interactions play a prominent role during intestinal translocation of an acylated peptide. Acylation benefits permeation for shorter and medium chains due to increased membrane interactions, however, for longer chains insertion in the membrane becomes dominant and hinders translocation, i.e. the peptides get ‘stuck’ in the cell membrane. Applying a transcellular absorption enhancer increases the dynamics of membrane insertion and detachment by fluidizing the membrane, thus facilitating its effects primarily on membrane associated peptides.

Fatty acids as therapeutic auxiliaries for oral and parenteral formulations

Many drugs have decreased therapeutic activity due to issues with absorption, distribution, metabolism and excretion. The co-formulation or covalent attachment of drugs with fatty acids has demonstrated some capacity to overcome these issues by improving intestinal permeability, slowing clearance and binding serum proteins for selective tissue uptake and metabolism. For orally administered drugs, albeit at low level of availability, the presence of fatty acids and triglycerides in the intestinal lumen may promote intestinal uptake of small hydrophilic molecules. Small lipophilic drugs or acylated hydrophilic drugs also show increased lymphatic uptake and enhanced passive diffusional uptake. Fatty acid conjugation of small and large proteins or peptides has exhibited protracted plasma half-lives, site-specific delivery and sustained release upon parenteral administration. These improvements are most likely due to associations with lipid-binding serum proteins, namely albumin, LDL and HDL. These molecular interactions, although not fully characterized, could provide the ability of using the endogenous carrier systems for improving therapeutic outcomes.

Peptide delivery using phospholipid micelles

Peptide based drugs are an important class of therapeutic agents but their development into commercial products is often hampered due to their inherent physico-chemical and biological instabilities. Phospholipid micelles can be used to address these delivery concerns. Peptides self-associate with micelles that serve to thwart the aggregation of these biomolecules. Self-association with micelles does not modify the peptide chemically; therefore the process does not denature or compromise the bioactivity of peptides. Additionally, many amphiphilic peptides adopt α-helical conformation in phospholipid micelles which is not only the most favorable conformation for receptor interaction but also improves their stability against proteolytic degradation, thus making them long-circulating. Furthermore, the nanosize of micelles enables passive targeting of peptides to the desired site of action through leaky vasculature present at tumor and inflamed tissues. All these factors alter the pharmacokinetic and biodistribution profiles of peptides therefore enhance their efficacy, reduce the dose required to obtain a therapeutic response and prevent adverse effects due to interaction of the peptide with receptors present in other physiological sites of the body. These phospholipid micelle based peptide nanomedicines can be easily scaled-up and lyophilized, thus setting the stage for further development of the formulation for clinical use. All things considered, it can be concluded that phospholipid micelles are a safe, stable and effective delivery option for peptide drugs and they form a great promise for future peptide nanomedicines.

Influence of acylation on the adsorption of GLP-2 to hydrophobic surfaces

Acylation of proteins with a fatty acid chain has proven useful for prolonging the plasma half-lives of proteins. In formulation of acylated protein drugs, knowledge about the effect of acylation with fatty acids on the adsorption behaviour of proteins at interfaces will be valuable. The aim of this work was to study the effect of acylation on the adsorption of GLP-2 from aqueous solution to a hydrophobic surface by comparing the adsorption of the 3766 Da GLP-2 with that of a GLP-2 variant acylated with a 16-carbon fatty acid chain through a β-alanine linker. Adsorption of GLP-2 and acylated GLP-2 were studied with isothermal titration calorimetry, fixed-angle optical reflectometry and total internal reflection fluorescence. Furthermore, the effect of acylation of GLP-2 on the secondary structure was studied with Far-UV CD. Acylation was observed to have several effects on the adsorption of GLP-2. Acylation increased the amount of GLP-2 adsorbing per unit surface area and decreased the initial adsorption rate of GLP-2. Finally, acylation increased the strength of the adsorption, as judged by the lower fraction desorbing upon rinsing with buffer.

Structural requirements for a lipoamino acid in modulating the anticonvulsant activities of systemically active galanin analogues

Introduction of lipoamino acid (LAA), Lys-palmitoyl, and cationization into a series of galanin analogues yielded systemically active anticonvulsant compounds. To study the relationship between the LAA structure and anticonvulsant activity, orthogonally protected LAAs were synthesized in which the Lys side chain was coupled to fatty acids varying in length from C(8) to C(18) or was coupled to a monodispersed polyethylene glycol, PEG(4). Galanin receptor affinity, serum stability, lipophilicity (log D), and activity in the 6 Hz mouse model of epilepsy of each of the newly synthesized analogues were determined following systemic administration. The presence of various LAAs or Lys(MPEG(4)) did not affect the receptor binding properties of the modified peptides, but their anticonvulsant activities varied substantially and were generally correlated with their lipophilicity. Our results suggest that varying the length or polarity of the LAA residue adjacent to positively charged amino acid residues may effectively modulate the antiepileptic activity of the galanin analogues.

Reversible lipidization of somatostatin analogues for the liver targeting

Tyr(3)-octreotide (TOC), a somatostatin analogue, is reversibly lipidized for passive delivery to the liver with the aim of increasing its association with hepatocytes. The reversibly lipidized TOC (REAL-TOC) was formed by the conjugation of the N-palmitoyl cysteinyl moiety to the cysteinyl residues of reduced TOC through disulfide linkages and characterized by matrix-assisted laser desorption/ionization (MALDI)-time of flight (TOF) analysis. The measured mass of REAL-TOC (M+H)(+) is 1752.31 Da (calculated mass: 1752.78), confirming that two molecules of N-palmitoyl cysteines are linked to TOC via disulfide bonds. TOC and REAL-TOC were radioiodinated and administered to mice. Their biodistribution and intrahepatic distribution were subsequently investigated. The area under the curve (AUC) of (125)I-REAL-TOC in the liver was 3.8-fold greater than that of (125)I-TOC, with 20.5% and 5.8% of the injected dose (ID)/g of (125)I-REAL-TOC remaining in the liver at 2 and 24h post injection, respectively. Within the liver, TOC was primarily distributed to parenchymal cells (PC). Nevertheless, TOC was quickly excreted out and only 2.4% ID per 100mg protein remained in the PC at 2h post injection. (125)I-REAL-TOC was retained in PC for up to 2h with a constant concentration of around 6% ID/100mg protein. (125)I-REAL-TOC was also highly associated with nonparenchymal cells (NPC) at significantly higher levels than (125)I-TOC at 10min, 1h and 2h post injection. Since somatostatin analogues have been evaluated for treating late-stage hepatocellular carcinoma (HCC), the reversibly lipidized conjugates may possess enhanced therapeutic efficacy due to the liver-targeting effect.

Use of the Biopharmaceutical Classification System in early drug development

The Biopharmaceutics Classification System (BCS) is not only a useful tool for obtaining waivers for in-vivo bioequivalence studies but also for decision making in the discovery and early development of new drugs. Measurement of solubility and permeability in the discovery/development settings is described. These data can be utilized for the preliminary BCS classification of pipeline compounds. A decision tree is described in the prioritization of salt and polymorph screening studies prior to in vivo studies in animals. For BCS class 1 and 3 compounds, polymorphism is less likely to impact on bioavailability. The polymorph screening study may be postponed after animal studies. The BCS classification can also be used in the design of animal and human formulations. A BCS-based animal formulation development decision tree is presented. A compound is triaged based on a series of decision points into one of the five formulation strategies. Human formulation has different requirements than animal formulation. A comparison between animal and human formulation strategies is presented. In conclusion, for non-BCS 1 compounds, the right-first-time polymorph and formulation selection ensures consistent pharmacokinetic performance and avoids bridging BA/BE studies. It is in line with FDA's initiative to reduce R&D cycle time through quality by design for pharmaceutical products.

The development of molecular clamps as drugs

Some enzymes catalyze the modification of an ensemble of substrates in vivo and, as a consequence, are not ideal targets for active-site-directed drugs. One solution to inhibiting such multisubstrate enzymes would be a drug that binds tightly to only one substrate, which prevents the binding of that substrate to the enzyme. Ideally, such a drug (called a molecular clamp, a molecular forcep or a molecular tweezer) would prevent the enzymatic processing of only the targeted substrate. This would enable the enzyme to function normally on all other substrates. Here, we review the unique steady-state kinetic features of molecular clamp inhibition, identify potential targets for molecular clamp inhibition, and discuss problems for the therapeutic use of molecular clamps.

Determination of long-acting release octreotide, an octapeptide analogue of somatostation, in human plasma by liquid chromatography/tandem mass spectrometry

A sensitive and selective method for the determination of long-acting released octreotide in human plasma has been developed based on liquid chromatography/tandem mass spectrometry (LC/MS/MS). Octreotide and the internal standard, triptorelin, were precipitated from the matrix, washed with dichloromethane and subsequently separated by reversed-phase high-performance liquid chromatography (HPLC) employing a 1% formic acid/methanol gradient system. Detection was by electrospray ionization mass spectrometry in the positive ion mode using multiple-reaction monitoring. The assay was linear in the concentration range 0.0500-50.0 ng/mL with intra- and inter-day precision (as relative standard deviation) of <2.95% and <8.37%, respectively. The limit of detection was 0.0200 ng/mL. The method was applied to a pharmacokinetic study of long-acting released octreotide in healthy volunteers given an intramuscular injection containing 20 mg octreotide.

Aqueous-Soluble, Non-Reversible Lipid Conjugate of Salmon Calcitonin: Synthesis, Characterization and In Vivo Activity

PURPOSE

A novel, non-reversible, aqueous-based lipidization strategy with palmitic acid as a model lipid was evaluated for conjugation with salmon calcitonin (sCT).

MATERIALS AND METHODS

A water-soluble epsilon-maleimido lysine derivative of palmitic acid was synthesized from reaction of palmitic acid N-succinimidyl ester and epsilon-maleimido lysine. The latter was generated from reaction of alpha-Boc-lysine and methylpyrrolecarboxylate, with subsequent deprotection of the Boc group. The palmitic derivative was further conjugated with sCT via a thio-ether bond to produce Mal-sCT in aqueous solution. The identity and purity of Mal-sCT was confirmed by Electrospray Ionisation Mass spectrometry (ESI-MS) and HPLC.

RESULTS

Yield of Mal-sCT was 83%. Dynamic light scattering and circular dichroism data suggested that Mal-sCT presented as a stable helical structure in aqueous solutions of varying polarity, with a propensity to aggregate at concentrations above 11 microM. Cellular uptake of Mal-sCT was twice that of sCT in the Caco-2 cell model, and the conjugate was more resistant to liver enzyme degradation. Mal-sCT exhibited comparable hypocalcemic activity to sCT when administered subcutaneously in the rat model at sCT equivalent dose of 0.114 mg/kg. Peroral Mal-sCT, however, produced variability in therapeutic outcome. While four out of six rats did not respond following intragastric gavage with Mal-sCT, two rats showed significantly suppressed plasma calcium levels (approximately 60% of baseline) for up to 10 h.

CONCLUSION

A novel non-reversible, water-soluble lipid conjugate of sCT was successfully synthesized that showed (1) different aggregation behavior and secondary structure, (2) improved enzymatic stability and cellular uptake, and (3) comparable hypocalcemic activity in vivo compared to sCT.