Pharmacokinetic disposition of polyethylene glycol-modified salmon calcitonins in rats

Pulmonary Delivery of the Kv1.3-Blocking Peptide HsTX1[R14A] for the Treatment of Autoimmune Diseases

HsTX1[R14A] is a potent and selective Kv1.3 channel blocker peptide with the potential to treat autoimmune diseases. Given the typically poor oral bioavailability of peptides, we evaluated pulmonary administration of HsTX1[R14A] in rats as an alternative route for systemic delivery. Plasma concentrations of HsTX1[R14A] were measured by liquid chromatography coupled with tandem mass spectrometry in rats receiving intratracheal administration of HsTX1[R14A] in solution (1-4 mg/kg) or a mannitol-based powder (1 mg/kg) and compared with plasma concentrations after intravenous administration (2 mg/kg). HsTX1[R14A] stability in rat plasma and lung tissue was also determined. HsTX1[R14A] was more stable in plasma than in lung homogenate, with more than 90% of the HsTX1[R14A] remaining intact after 5 h, compared with 40.5% remaining in lung homogenate. The terminal elimination half-life, total clearance, and volume of distribution of HsTX1[R14A] after intravenous administration were 79.6 ± 6.5 min, 8.3 ± 0.6 mL/min/kg, and 949.8 ± 71.0 mL/kg, respectively (mean ± SD). After intratracheal administration, HsTX1[R14A] in solution and dry powder was absorbed to a similar degree, with absolute bioavailability values of 39.2 ± 5.2% and 44.5 ± 12.5%, respectively. This study demonstrated that pulmonary administration is a promising alternative for systemically delivering HsTX1[R14A] for treating autoimmune diseases.

Synthesis, characterization and evaluation of bone targeting salmon calcitonin analogs in normal and osteoporotic rats

In order to assess the therapeutic efficacy of an antiresorptive drug with imparted bone targeting potential using bisphosphonate (BP) conjugation and an improved pharmacokinetic profile using PEGylation, we synthesized, characterized and evaluated in vivo efficacy of bone-targeting PEGylated salmon calcitonin (sCT) analog (sCT-PEG-BP). sCT-PEG-BP was compared with non-PEGylated bone targeting sCT analog (sCT-BP) and unmodified, commercially available sCT. sCT-PEG-BP conjugates were characterized by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis. The effect of PEG-BP or BP upon sCT secondary structure was examined by Circular Dichroism and sCT-PEG-BP was evaluated for in vitro bone mineral Hydroxyapatite (HA) binding ability and calcium salts specificity using a binding assay for bone HA and several calcium salts. Anti-calcitonin antibody binding ability of these analogs was determined using enzyme-linked immunosorbent assay (ELISA), by reacting bone targeting sCT analogs with calcium phosphate coated Osteologic® plates and detecting the bound sCT using anti-sCT antibody. Potential cytotoxicity of these compounds was evaluated in monocytic RAW 264.7 cells, and sCT bioactivity was evaluated using an in vitro intracellular cAMP stimulation assay in human T47D breast cancer cells. Finally, in vivo efficacy of each compound was evaluated by determining the plasma levels of calcium after s.c. administration in normal rats, and in a rat model of Osteoporosis, secondary to ovariectomy (OVX). In vivo micro-computed tomography (micro-CT) was used to temporally map and quantify alterations in bone volume and bone mineral density (BMD) in the same animals at 1, 4, 8 and 12 weeks after OVX surgery. Sixteen 6 week old virgin female rats underwent OVX surgery followed by the daily s.c. injection of 2.5IU/kg/day sCT or equivalent analogs, and compared to four sham-operated, placebo treated control rats. Our results showed the chemical coupling of PEG-BP or BP to sCT altered its secondary structure without altering its antibody binding ability. sCT analogs retained strong sCT bioactivity, were non-toxic to RAW 264.7 cells in culture and elicited a comparable hypocalcemic effect to that of unmodified sCT in normal rats. Compared to marketed unmodified sCT, sCT-PEG-BP showed significantly improved efficacy in terms of preserving bone volume, BMD and trabecular micro-architecture in osteoporotic rats at the initial dose tested. Bisphosphonate-mediated targeting of PEGylated sCT to bone represents a new class of targeted antiresorptive compounds that has not previously been attempted.

Preparation and characterization of salmon calcitonin-biotin conjugates

This study was performed to prepare and characterize the biotinylated Salmon calcitonin (sCT) for oral delivery and evaluate the hypocalcemic effect of biotinylated-sCTs in rats. Biotinylated sCTs was characterized by using high performance liquid chromatography (HPLC) and MALDITOF-MS. The effect of biotinylation on permeability across Caco-2 cell monolayers was examined. Their hypocalcemic effect was determined in rats. Mono- and di-bio-sCTs were separated by reverse phase HPLC. The molecular weights of mono-bio-sCT and di-bio-sCT were determined to be 3,660.5 and 3,900.2 Da, respectively. The permeability of biotinylated-sCTs across Caco-2 cell monolayers was observed with a significant enhancement compared with sCT. Intrajejunal (ij) administration of mono-bio-sCT and di-bio-sCT resulted in sustained reduction in serum calcium levels, with a maximum reduction (% max(d)) of 21.6% and 30% after 4 h and 6 h of application, respectively. The biotin conjugation of sCT may be a promising strategy for increasing the oral bioavailability of sCT and achieving sustained calcium-lowering effects.

Improved intrapulmonary delivery of site-specific PEGylated salmon calcitonin: optimization by PEG size selection

The purpose of this study was to demonstrate the biological potentials of PEGylated salmon calcitonin (PEG-sCT) derivatives administered intratracheally and their dependences on PEG Mw (1, 2, 5 kDa). Initially, three different PEG-sCT derivatives were site-specifically synthesized by attaching PEG to the Lys(18)-amine. In an attempt to examine the pulmonary feasibilities of these derivatives, the following evaluations were undertaken to determine their; (i) proteolytic resistances to pulmonary enzymes, (ii) bioactivities, and (iii) pulmonary pharmacokinetic and pharmacologic profiles. The results obtained showed that the pulmonary stabilities and pharmacokinetic properties of these derivatives were greatly improved by increasing PEG Mw. PEG-sCTs had 10.5-, 40.1-, and 1066.0-fold greater stabilities than that of sCT in rat lung homogenates. Moreover, all pharmacokinetic parameters (AUC(inf), C(max), t(1/2), and others) of these derivatives in endotracheally cannulated rats were significantly improved by PEGylation. Specifically, C(max) values increased on increasing PEG Mw, i.e., 78.1+/-21.1, 102.9+/-9.1, and 115.2+/-5.7 for 1, 2, 5 kDa, respectively, vs. 54.8+/-3.9 ng/mL for sCT. Their circulating t(1/2) values also increased to 53.9+/-6.0, 100.7+/-21.7, and 119.4+/-13.7 min, respectively, vs. 34.6+/-7.6 min for sCT. Despite having the best properties, Lys(18)-PEG(5k)-sCT was found to have significantly lower hypocalcemic efficacy than other PEG-sCTs, probably due to its reduced intrinsic bioactivity ( approximately 30% vs. sCT). Rather, Lys(18)-PEG(2k)-sCT showed the most promising pulmonary potential because of its well-preserved bioactivity (>80% of sCT). Taken together, our findings suggest that the site-specific substitution to peptides like sCT with a PEG of an appropriate size offers optimized therapeutic potential by dual advantages, i.e., (i) increased proteolytic stability and (ii) extended circulating half-life in terms of intrapulmonary delivery.

Pharmacokinetic consequences of pegylation

Pegylation, generally described as the molecular attachment of polyethylene glycols (PEGs) with different molecular weights to active drug molecules or surface treatment of drug-bearing particles with PEGs, is one of the most promising and extensively studied strategies with the goal of improving the pharmacokinetic behavior of the therapeutic drugs. A variety of PEGs, both linear and branched, with different molecular weights have been exploited successfully for use in this procedure in the form of reactive PEG species. Both reversible and irreversible PEG-drug conjugates have been prepared with relative advantages/disadvantages. The main pharmacokinetic outcomes of pegylation are summarized as changes occurring in overall circulation life-span, tissue distribution pattern, and elimination pathway of the parent drug/particle. Based on these favorable pharmacokinetic consequences leading to desired pharmacodynamic outcomes, a variety of proteins/peptides as well as small molecule drugs have been pegylated and evaluated successfully. Also a number of corresponding products have been approved by the U.S. FDA for specific clinical indications and some others are underway. In this article, the chemistry, rationale, strategies, pharmacokinetic outcomes, and therapeutic possibilities of pegylated drugs are reviewed with pharmacokinetic aspects presented with more details.

Improved intestinal delivery of salmon calcitonin by Lys18-amine specific PEGylation: stability, permeability, pharmacokinetic behavior and in vivo hypocalcemic efficacy

Peptides like salmon calcitonin (sCT) are subjected to aggressive proteolytic attack by various intestinal enzymes, and fractions that enter the systemic circulation via the intestinal route are rapidly inactivated by tissue accumulation and glomerular filtration. Here, we describe the beneficial effects of the Lys(18)-amine specific PEGylation of sCT on the intestinal delivery of sCT. Two key properties were enhanced by the PEGylation process: (i) the resistance of sCT to intestinal enzymes and (ii) the systemic clearance of sCT that had entered the circulation. Initially, we evaluated the cAMP-secreting activities of PEG(2K)-sCT isomers substituted at Cys(1)-, Lys(11)- or Lys(18)-amine position in T47D cells, and found that sCT PEGylated at Lys(18)-amine (Lys(18)-PEG(2K)-sCT) had the highest bioactivity. We then investigated the stability of Lys(18)-PEG(2K)-sCT in the presence of intestinal enzymes, its abilities to traverse the intestinal membrane, its pharmacokinetic behavior and in vivo hypocalcemic efficacy. Results show that Lys(18)-PEG(2K)-sCT has significantly increased resistance to pancreatic peptidases and brush-border peptidases. Despite the molecular size increase caused by PEGylation, Lys(18)-PEG(2K)-sCT was found to have an intestinal permeability similar to that of unmodified sCT (p>0.59) over an apical concentration range 12.5-100 microM in a Caco-2 cell monolayer transport system. In particular, tissue distribution results showed that (125)I-labeled Lys(18)-PEG(2K)-sCT markedly resists liver accumulation and glomerular filtration; levels were reduced by 75% and 50% vs. sCT. Finally, the hypocalcemic efficacy of intestinally administered Lys(18)-PEG(2K)-sCT, measured as total serum calcium in a rat model, was 5.8 and 3.0 times that of sCT at 100 and 200 IU/kg (p<0.025). Our findings suggest that this site-specific conjugation of peptides with PEG of proper size enhances pharmacokinetic properties by increasing their abilities to resist both proteolysis and systemic clearance without significantly reducing their membrane permeabilities or bioactivities. We believe that this concept, namely, dual effects by PEGylation, has great potential value because it presents a practical means of enhancing the efficacies of the peroral/intestinal pharmacologic route.

Nasal absorption and pharmacokinetic disposition of salmon calcitonin modified with low molecular weight polyethylene glycol

This study was performed to examine the absorption potential of polyethylene glycol (PEG)-modified salmon calcitonin (sCT) in rats administered via the nasal route. Previous studies have used relatively high molecular weight polyethylene glycols (e.g., > or =5000 daltons) for PEG-modification of sCT to provide increased metabolic stability and biological half-life. Unlike these studies, the present study utilized a low molecular weight succinimidyl-propionated monomethoxy-poly(ethylene glycol) (MW 2000). It was hypothesized that the potential for membrane transport would not be significantly altered due to a relatively small increase in the molecular size while the metabolic stability would be enhanced due to resistance to proteolytic degradation. After PEG-modification of sCT, the mono-PEG positional isomer (mono-PEG2k-sCT) was separated from di-PEG2k-sCT, tri-PEG2k-sCT, and unmodified sCT by size exclusion chromatography. The mono-PEG2k-sCT and unmodified sCT were radioiodinated, and the resulting 125I-sCT and 125I-mono-PEG2k-sCT were separated from free iodine by RP HPLC and confirmed by MALDI-TOF MS. The 125I-sCT and 125I-mono-PEG2k-sCT were administered to rats via the nasal route, and serial blood, tissue, and urine samples were taken for up to 36 h for the determination of radioactivity. Mono-PEG2k-sCT exhibited significantly increased AUC (20,638 vs. 3,650 ng.min/ml), tmax (520 vs. 77 min), and t1/2,lambdaz (923 vs. 199 min) compared with unmodified sCT. This study demonstrates that mono-PEG2k-sCT is absorbed systemically when given by the intranasal route, exhibiting altered absorption kinetics compared with unmodified sCT.

Covalent polymer-drug conjugates

In this work, polymer-drugs conjugates used as drug delivery systems (DDS) are revised attending to their chemical conjugation. Namely, the classification of this type of DDS is based on the conjugation sites of the reactive groups (i.e., via end groups or pendant polymer groups). Advantages and limitations of these types of DDS are discussed through representative examples of polymer-drugs and polymer-proteins conjugates recently developed.

Plasma expansion by polyethylene-glycol-modified albumin

Systemic inflammatory response conditions are associated with capillary leak and haemodynamic compromise. Fluid resuscitation to reverse the ensuing hypovolaemia is, however, complicated by the decreased endothelium reflection coefficient to albumin and other colloids. We developed PEG–Alb (albumin covalently linked to polyethylene glycol) as a potential resuscitative agent. PEG was covalently linked to human albumin at multiple sites on the protein. The modified protein was heterogeneous when examined by SDS/PAGE, size-exclusion chromatography and SELDI–TOF MS (surface-enhanced laser-desorption ionization–time of flight MS). Based on size-exclusion chromatography and osmotic pressure data, the effective volume of PEG–Alb is increased 13- to 16-fold compared with unmodified albumin. In an LPS (lipopolysaccharide) model of shock, rats treated with PEG–Alb showed better blood pressure, lower Hct (haematocrit) consistent with haemodilution and less lung injury than rats treated with unmodified albumin or saline. In a CLP (caecal ligation and puncture) model of sepsis, PEG–Alb was more effective than albumin or saline in maintaining blood pressure and in decreasing Hct. When fluorescein-labelled PEG–Alb and Texas Red-labelled albumin were administered to rats with LPS- or CLP-induced shock, PEG–Alb was retained within blood vessels, whereas albumin extravasates into the interstitial space. Based on these data, PEG–Alb appears to be retained within blood vessels in models of capillary leak. PEG–Alb may ultimately be effective in the clinical treatment of shock associated with capillary leak.

Calcitonin: physiological actions and clinical applications

Calcitonin (CT) was first reported as a hypocalcemic principle, initially thought to originate from the parathyroid gland, a view subsequently corrected to an origin from parafollicular C-cells. Human CT is a 32 amino acid peptide with an N-terminal disulphide bridge and a C-terminal prolineamide residue, shown to potently inhibit bone resorption. More recent studies have demonstrated that this may take place through a direct osteoclastic action. A number of osteoclast CT receptors have subsequently been characterized and particular receptor regions necessary for ligand binding and intracellular signaling identified. Its potent anti-resorptive effect has led to its use in treating Paget's bone disease, osteoporosis, hypercalcaemia and osteogenesis imperfecta. This review summarises some key aspects of its synthesis, structure and its actions at the cellular and molecular levels, and leads on to its therapeutic uses that have emerged since its discovery as well as possibilities for future clinical applications.

Effective drug delivery by PEGylated drug conjugates

The current review presents an update of drug delivery using poly(ethylene glycol) (PEG), that focuses on recent developments in both protein and organic drugs. Certainly the past 10 years has resulted in a renaissance of the field of PEG drug conjugates, initiated by the use of higher molecular weight PEGs (M(w)>20,000), especially 40,000 which is estimated to have a plasma circulating t(1/2) of approximately 10 h in mice. This recent resuscitation of small organic molecule delivery by high molecular weight PEG conjugates was founded on meaningful in vivo testing using established tumor models, and has led to a clinical candidate, PEG-camptothecin (PROTHECAN), an ester based prodrug currently in phase II trials. Additional applications of high molecular weight PEG prodrug strategies to amino containing drugs are presented: similar tripartate systems based on lower M(w) PEG and their use with proteins is expounded on. The modification of a benzyl elimination tripartate prodrug specific for mercaptans is presented, and its successful application to 6-mercaptopurine giving a water soluble formulation is discussed. Recent novel PEG oligonucleotides and immunoconjugates are also covered. Clinical results of FDA approved PEGylated proteins are also presented.

Calcitonin: the other thyroid hormone

Calcitonin was originally discovered as a hypocalcemic factor synthesized by thyroid parafollicular C cells. Early experiments demonstrated that calcitonin inhibited bone resorption and decreased calcium efflux from isolated cat tibiae and subsequent histologic and culture studies confirmed the osteoclast as its major site of action. Its potent antiresorptive effect and analgesic action have led to its clinical use in treatment of Paget's bone disease, osteoporosis, and hypercalcemia of malignancy. This review surveys the cellular and molecular basis of these physiologic and clinical actions.

Simple liquid chromatography-electrospray ionization mass spectrometry method for the routine determination of salmon calcitonin in serum

A simple liquid chromatography-electrospray ionization mass spectrometry (LC-ESI-MS) method was developed for the quantification of salmon calcitonin (sCT) in serum. Serum samples from rats and dogs were deproteinized and freeze-dried. The residue was then reconstituted with 57% acetonitrile in water containing 0.1% trifluoroacetic acid and 0.005% benzalkonium chloride. A 20-microl aliquot of the reconstituted solution was injected onto a polymer based RP-C18 column, The outlet was connected to an ion-trap mass spectrometer equipped with an ESI source, and spectra were recorded in a positive-ion, selected-ion monitoring mode. The limit of quantification of the method was 10 ng/ml. Biexponential curves were observed for the temporal serum concentration of sCT following intravenous administration of sCT to rats (100 microg/kg) and dogs (250 microg/kg), resulting in reasonable pharmacokinetic parameters. The present method appears applicable to routine analysis of serum sCT in pharmacokinetic studies with good selectivity, accuracy and precision.

Forty years of calcitonin--where are we now? A tribute to the work of Iain Macintyre, FRS

Calcitonin was discovered as a hypocalcemic principal that was initially thought to originate from the parathyroid gland. This view was corrected subsequently, and an origin from the thyroid C cells was documented. The purification and sequencing of various calcitonins soon followed. Calcitonin is a 32-amino-acid-long peptide with an N-terminal disulfide bridge and a C-terminal prolineamide residue. The peptide was shown to potently inhibit bone resorption; however, a direct osteoclastic action of the peptide was confirmed only in the early 1980s. Several osteoclast calcitonin receptors have subsequently been cloned and sequenced. Specific regions of the receptor necessary for ligand binding and intracellular signaling through cyclic AMP and calcium have been identified through systematic deletion mutagenesis and chimeric receptor studies. Calcitonin's potent antiresorptive effect has led to its use in treating Paget's disease of bone, osteoporosis, and hypercalcemia. This review retraces key aspects of the synthesis and structure of calcitonin, its cellular and molecular actions, and its therapeutic uses as they have emerged over the 40 years since its discovery. The review also examines the implications of these findings for future clinical applications as a tribute to early workers to whom credit must be given for creation of an important and expanding field. Notable are the new approaches currently being used to enhance calcitonin action, including novel allosteric activators of the calcitonin receptor, modulation of the release of endogenous calcitonin by calcimimetic agents, as well as the development of oral calcitonins.