An in vivo/in vitro comparison with a leuprolide osmotic implant for the treatment of prostate cancer

An osmotically driven implantable system was designed and characterized for the delivery of leuprolide over a year-long duration. Leuprolide has been used in the treatment of prostate cancer since the 1980s. The DUROS implant consists of a titanium alloy cylinder, measures 4 mm in diameter by 45 mm in length and holds approximately 150 microl of formulation. Stability studies indicated that leuprolide was stable, as a solution formulation in DMSO, for several years at 37 degrees C. In vitro release rate testing, at weekly intervals, showed zero-order delivery for 1 year. DUROS implants demonstrated release rates that were reproducible and similar to one another after storage at 25 degrees C for 18 months prior to testing. In vivo studies, with implants placed subcutaneously, revealed delivery rates comparable to those observed under in vitro conditions. Leuprolide stability was also comparable between in vivo and in vitro conditions. Steady leuprolide serum levels produced by the implant resulted in the desired pharmacodynamic efficacy endpoint of testosterone suppression, both in canines and in humans. The good agreement between in vivo/in vitro delivery rates was as expected for a delivery system based on the principles of osmosis.

Characterization of protein and peptide stability and solubility in non-aqueous solvents

Small molecule parenterals have often been formulated as solutions or suspensions in non-aqueous conditions, however, this technology has not found widespread use in the formulation of macromolecules. Formulation of proteins and peptides has primarily been achieved through aqueous solutions or reconstituted lyophilized cakes. The incorporation of non-aqueous techniques has been limited by the lack of general applicability. For example, prediction of solubility, chemical stability, conformational stability (unfolding/denaturation processes), and activity can be difficult. Therefore, macromolecule non-aqueous preformulation work must be performed on a case by case basis. In addition, only a few solvents are pharmaceutically acceptable. This article reviews the characterization of proteins and peptides in a variety of non-aqueous or co-solvent conditions (both acceptable and unacceptable for pharmaceutical applications), and discusses the applicability of non-aqueous conditions for increasing solubility, stability and activity.

Solution stability of salmon calcitonin at high concentration for delivery in an implantable system

Salmon calcitonin solutions (50 mg/mL and 100 mg/mL) were placed on stability at 37 degrees C for 1 year in a variety of solvent systems including water, ethanol, glycerol, propylene glycol (PG) and dimethyl sulfoxide (DMSO). Calcitonin degradation was monitored by RP-HPLC and size-exclusion chromatography. DMSO and pH 3.3 solutions provided optimum stability. Conformational stability was also monitored by FTIR over the 1 year time course and compared with chemical and physical stability. After 12 months at 37 degrees C, four major conformations were observed: a beta-sheet conformation (pH 3.3, pH 5.0, 70% DMSO and 70% glycerol), an aggregate conformation (pH 7.0 water), a strong alpha-helical conformation (70% EtOH, 70% PG) and a weak alpha-helical conformation (100% DMSO). No correlation between structure and chemical stability was observed in which both the beta-sheet structure (pH 3.3, water) and a loose alpha-helical structure (100% DMSO) demonstrated good stability. However, some correlation was observed between structure and physical stability, where co-solvents inducing an alpha-helical structure resulted in a decrease in gelation. These two structural states associated with improved stability and minimal gelation, indicated that gelation can be reduced or eliminated by the use of pharmaceutically acceptable co-solvents. Finally, salmon calcitonin (50 mg/mL) was formulated in 100% DMSO and delivered from a DUROS implant over 4 months. Delivery at a target dose of 18 microg/day calcitonin at 37 degrees C was confirmed.

Effect of peptide concentration and temperature on leuprolide stability in dimethyl sulfoxide

The effects of temperature and concentration on leuprolide degradation in dimethyl sulfoxide (DMSO) were explored. Leuprolide degradation products were analyzed by reverse phase high-performance liquid chromatography (RP-HPLC), size exclusion chromatography (SEC) and structurally characterized by mass spectrometry. Leuprolide solution stability in DMSO was characterized at 50, 100, 200, 400 mg/ml at 37-80 degrees C for 2 months to 3 years. Leuprolide degradation products were identified by mass spectrometry and could generally be attributed to isomerization, hydrolysis, oxidation, or aggregation. The hydrolytic degradation products consisted primarily of backbone cleavage C-terminal to Trp(3), Ser(4), Tyr(5), Leu(6) and Leu(7), and oxidation of Trp(3) and beta-elimination of Ser(4) were identified. Leuprolide degradation at 50 degrees C, 65 degrees C and 80 degrees C proceeded in an exponential fashion (E(a)=22. 6+/-1.2 kcal/mol); however, leuprolide degradation plateau'd after approximately 6 months at 37 degrees C. Upon closer examination, degradation product peak areas were seen to vary with temperature. For example, aggregation products did not increase with time at 37 degrees C, but aggregation peak intensities increased sharply with time at 80 degrees C. Increasing the temperature also increased the proportion of leuprolide degrading via isomerization/hydrolytic pathways, and decreased the proportion degrading via oxidation. These variations suggested that solvent dielectric, free H(+) in an aprotic solvent, oxygen solubility, impurities and residual moisture may play a role. Leuprolide solubilized in DMSO yields adequate stabililty for a 1 year implantable osmotic delivery system, where use of a dry aprotic solvent results in conditions similar to solid state stability.

Characterization and comparison of leuprolide degradation profiles in water and dimethyl sulfoxide

The effect of solvent on the rate of leuprolide degradation and on the structure of the degradation products was explored. Leuprolide solutions (370 mg/mL) were prepared in water and dimethyl sulfoxide (DMSO) for delivery in DUROS osmotic implants. Both solvent systems demonstrated better than 90% stability after 1 year at 37 degrees C, where the DMSO formulation afforded better stability than the aqueous formulation and was used in subsequent clinical trials. The rate of leuprolide degradation in DMSO was also observed to accelerate with increasing moisture content, indicating that the aprotic solvent minimized chemical degradation. Interestingly, leuprolide degradation products varied with formulation vehicle. The proportions of leuprolide degradation products observed to form in water and DMSO at 37 degrees C were hydrolysis > aggregation > isomerization > oxidation and aggregation > oxidation > hydrolysis > isomerization, respectively. Specifically, more N-terminal hydrolysis and acetylation were observed under aqueous conditions, and increased Trp oxidation and Ser beta-elimination were seen under non-aqueous conditions. Furthermore, the major chemical degradation pathway changed with temperature in the DMSO formulation (decreasing oxidation with increasing temperature), but not in the aqueous formulation.

Use of Caco-2 cells and LC/MS/MS to screen a peptide combinatorial library for permeable structures

The transepithelial transport of a synthetic peptide combinatorial library containing 375,000 individual peptides was assessed using Caco-2 cell monolayers in order to screen for permeability and deliverability. A series of 150 pools, each containing 2500 tripeptide sequences, were applied to the apical side of Caco-2 monolayers. Basolateral side samples were collected after 4 h and screened by capillary high-pressure liquid chromatography. The majority of pools showed no permeable species, due to low solubility, limited permeability and extensive metabolism. Several pools contained permeable structure, and transport proved reproducible with passage number and time. Permeable structures were identified by liquid chromatography/mass spectrometry/mass spectrometry (LC/MS/MS). To discriminate between isobaric structures, several tripeptides were resynthesized and tested as discrete compounds. For example, 1-2% D-Phe-D-Ala-D-Ser-OH was transported across the Caco-2 cell monolayer with a Papp value of 0.35-0.69 x 10(-6) cm/s, which is comparable with the permeability of amino acids (Leu, Papp = 0.30 x 10(-6) cm/s) and dipeptides (L-Val-L-Val, Papp = 0.18 x 10(-6) cm/s) (Lennernas, H., Palm, K., Fagerholm, U., Artursson, P., 1996. Comparison between active and passive drug transport in human intestinal epithelial (Caco-2) cells in vitro and human jejunum in vivo. Int. J. Pharm. 127, 103-107; Tamura, K., Bhatnagar, P.K., Takata, J.S., Lee, C.P., Smith, P.L., Borchardt, R.T., 1996. Metabolism, uptake, and transepithelial transport of the diastereomers of Val-Val in the human intestinal cell line Caco-2. Pharm. Res. 13, 1213-1218). These studies demonstrate the techniques used to screen combinatorial libraries for permeability across Caco-2 cells and structurally identify the resulting compounds. Such methodology can be of importance in the achievement of structure-permeability relationships, useful in the design of pharmaceutically bioavailable drugs.

Effect of gelation on the chemical stability and conformation of leuprolide

PURPOSE

The purpose of this study was to characterize the conformation, aggregation, and stability of leuprolide on gelation.

METHODS

Infrared spectra (FTIR) of leuprolide solutions and gels were collected in water, propylene glycol (PG), dimethyl sulfoxide (DMSO), and trifluoroethanol (TFE). Leuprolide solution and gel stability data were obtained by SEC and RP-HPLC.

RESULTS

Leuprolide was induced to gel with increasing peptide concentration, introduction of salts, and gentle agitation. Leuprolide dissolved in water (400 mg/ml) demonstrated FTIR spectra consisting of two major bands of equal intensity at 1615 cm(-1) and 1630 cm(-1), similar to inter- and intra-molecular beta-sheet structure in proteins. When samples were gently agitated for 24 hours at 25 degrees C, the formulation was observed to change from a viscous liquid to an opaque gel with a concomitant shift in infrared spectra from the equal intensity bands to mostly 1630 cm(-1), indicating a shift to a preferred beta-sheet structure. Incubation of leuprolide with 20-200 mM salts at 25 degrees C and 37 degrees C also produced gels ranging from clear to cloudy and stringy white precipitates. The gel and precipitate were marked by a shift of the predominant beta-sheet band to 1630 cm(-1) and 1615 cm(-1), respectively. Leuprolide was also observed to gel and/or precipitate in mixtures of water, PG or TFE, but not in DMSO.

CONCLUSIONS

Birefringence was noted in many of the firmer gels. Both solutions and gels demonstrated minimal dimer or trimer formation, with no larger order aggregates detected. The chemical stability profile of gelled leuprolide was similar to that of the non-gelled water formulation by RP-HPLC.

Precipitation of proteins in supercritical carbon dioxide

Supercritical CO2 was used as an antisolvent to form protein particles that exhibited minimal loss of activity upon reconstitution. Organic protein solutions were sprayed under a variety of operating conditions into the supercritical fluid, causing precipitation of dry, microparticulate (1-5 microns) protein powders. Three proteins were studied: trypsin, lysozyme, and insulin. Amide I band Raman spectra were used to estimate the alpha-helix and beta-sheet structural contents of native and precipitate powders of each protein. Analysis of the Raman spectral revealed minimal (lysozyme), intermediate (trypsin), and appreciable (insulin) changes in secondary structure with respect to the commercial starting materials. The perturbations in secondary structure suggest that the most significant event during supercritical fluid-induced precipitation involved the formation of beta-sheet structures with concomitant decreases of alpha-helix. Amide I band Raman and Fourier-transform infrared (FTIR) spectra indicate that higher operating temperatures and pressures lead to more extensive beta-sheet-mediated intermolecular interactions in the precipitates. Raman and FTIR spectra of redissolved precipitates are similar to those of aqueous commercial proteins, indicating that conformational changes were reversible upon reconstitution. These results suggest that protein precipitation in supercritical fluids can be used to form particles suitable for controlled release, direct aerosol delivery to the lungs, and long-term storage at ambient conditions.

Estimation of recombinant bovine somatotropin solubility by excluded-volume interaction with polyethylene glycols

PURPOSE

The potential to estimate protein solubilities, with limited protein, by excluded-volume interactions was evaluated using polyethylene glycols (PEG) and recombinant bovine Somatotropin (rbSt).

METHODS

Solutions of rbSt were prepared at concentrations significantly below saturation solubility. Subsequently, varying amounts of PEG were added to force protein precipitation. Following centrifugation, the protein concentration in the supernatant was assayed by reversed-phase HPLC, where a logarithmic relationship between solubility and % PEG was observed.

RESULTS

An apparent protein solubility in the absence of PEG was determined by extrapolation and compared well with values measured by conventional approaches. Slopes of log solubility versus % PEG curves were consistent with excluded-volume principles and depended on the molecular weight of the PEG used. Furthermore, the precipitation process proved to be reversible, allowing for recovery of intact protein. Solubility-pH profiles obtained in the presence of PEG greatly reduced the quantities of protein needed and compared favorably with profiles in the absence of PEG.

CONCLUSIONS

Thus, it appears feasible and practical, with certain limitations, to obtain solubility estimates of proteins by volume-exclusion methods with limited supplies of protein.

Synchronous luminescence: a new detection technique for multiple fluorescent probes used for DNA sequencing

A method fo nonradioactive DNA sequencing, which has been marketed commercially, uses four different fluorescent tags to label DNA fragments and fixed-wavelength excitation/fluorescence detection of the labels. This study presents an alternative method of detection based on synchronous luminescence (SL), in which both excitation and emission wavelengths are scanned simultaneously. This approach has proven in the past to have significant advantages over fixed-wavelength luminescence in the analysis and identification of fluorescent analytes. In this paper, the utility of synchronous excitation was investigated as a method for DNA sequencing with fluorescent tags. A laser-based SL instrument, recently developed in this laboratory, was used to resolve the spectra of a mixture of the dyes used in the fluorescence-based sequencing scheme. A preliminary limit of detection of 720 zeptomoles (10(-21) M) of fluorescein isothiocyanate dye in solution was achieved with this instrument. The results presented here suggest that the SL technique could result in an increase in sensitivity and a decrease in error rate of identification during DNA sequencing.

The mass spectrometry of helical unfolding in peptides

Two model peptides, melittin and a growth hormone releasing factor (GRF) analog, have been studied by mass spectrometry and tandem mass spectrometry during the course of their deuterium exchange. Both peptides are known from previous work to form α-helices in solution. When the peptides are exposed to deuterated solvents, their masses increase as deuterium atoms replace protons in the exchangeable sites of the peptides. The mass spectrometry results clearly indicate multiple populations of exchangeable protons: Some exchange very fast, and are presumably on the surface and not involved in hydrogen bonding; others exchange much more slowly, indicating that they are probably participating in hydrogen bonding.Tandem mass spectrometric experiments were conducted, and the masses of the product (fragment) ions were used to determine where in the peptide the deuterium atoms were incorporated. The results agree very well with NMR studies of the same peptides. Melittin appears as two helical segments with a kink around Pro-14. The GRF analog contains a single long helix, spanning almost the entire length of the peptide. The dynamics of the unfolding of the helices can also be explored by observing how the exchange progresses with time.

Effect of secondary structure on the rate of deamidation of several growth hormone releasing factor analogs

The objective of this study was to determine whether the rates of deamidation of Asn8 in selected growth hormone releasing factor (GRF) analogs were related to the peptide's secondary structures in solution. Bovine or human [Leu27]GRF(1-32)NH2 (both having Gly at position 15), [Ala15Leu27]bGRF(1-32)NH2 and [Pro15Leu27]bGRF(1-32)NH2 were used as model peptides. The peptide helical content (assessed by CD) increased with the increasing methanol concentration and was as follows: 7, 12 and 18% in 0% MeOH; 24, 48 and 52% in 40% MeOH; and 41, 77 and 81% in 80% MeOH for Pro15Leu27 bGRF(1-32)NH2, [Leu27]hGRF(1-32)NH2 and Ala15Leu27 bGRF(1-32)NH2, respectively. 2D NMR studies done in the presence of 40% CD3OH indicated more helical structure for the Ala15 analog as compared to [Leu27]hGRF(1-32)NH2. In both these peptides Asn8 was included in the helical region. In contrast, the lack of conformational information for the Pro15 analog indicated little helical structure around Asn8. The peptides' deamidation rates decreased and their half-lives increased with increasing MeOH concentrations. At 40% MeOH, the least helical Pro15 bGRF analog (t1/2 = 10.78 h) deamidated 1.5 and 2 times faster than its Gly15 (t1/2 = 15.74 h) and Ala15 (t1/2 = 21.53 h) counterparts, respectively. This study indicates that helical environment around Asn8 in GRF makes this residue less prone to deamidation.

Probing the helical content of growth hormone-releasing factor analogs using electrospray ionization mass spectrometry

A series of growth hormone-releasing factor analogs have been studied by both circular dichroism and electrospray ionization mass spectrometry (ESI/MS). The peptides are 32 residues long and are known to adopt a random-coil structure in aqueous solution but become increasingly helical as the proportion of organic solvent is increased. Deuterium exchange was observed as an increase in mass of the peptide, as measured by ESI/MS. Rates of exchange were measured and half-lives calculated for analogs containing amino acid substitutions designed to promote or discourage helix formation. Exchange was slower in peptides that are helical (as shown by circular dichroism) than in randomly coiled peptides. Solution conditions that favor helix formation also produced slower exchange rates. These studies suggest that ESI/MS can provide date about the extent and stability of helix formation.

Solution conformation of Leu27 hGRF(1-32)NH2 and its deamidation products by 2D NMR

The solution structure and helical content of a human growth hormone releasing factor analog, Leu27 hGRF(1-32)NH2 (hGRF), and its deamidation products Asp8 Leu27 hGRF(1-32)NH2 and isoAsp8 Leu27 hGRF(1-32)NH2, were determined by CD and 2D NMR. Chemical-shift assignments of 1H NMR resonances were made from DQFCOSY, HOHAHA and NOESY spectra, and qualitative secondary structure was determined from NOESY spectra. 2D NMR studies in aqueous MeOH showed the Asn8, Asp8 and isoAsp8 hGRF analogs to have significant alpha-helical character. However, the beta-linked isoAsp8 analog did not retain helical structure in the N-terminal region, most likely because of disruption of the hydrogen bonding pattern upon substitution of the extra methylene into the peptide backbone. The helical content, as determined by CD, was approximately 12% in 0% MeOH for all three peptides, and 77, 72 and 69% in 80% MeOH for the Asn8, Asp8 and isoAsp8 hGRF analogs, respectively. However, 2D NMR solution structure data indicated a decrease in helicity in the N-terminal region for the isoAsp8 analog when compared with the other two analogs. In the Asn8 and Asp8 hGRF analogs, the helix began at Asp3 or Ala4, while the isoAsp8 analog helix was disrupted until Arg11. The higher helicity value for the Asn8 peptide over the isoAsp8 analog may be associated with reported biological activity, where the in vitro activity decreased from 100 to 4 and < 1% for Asn8, Asp8 and isoAsp8 hGRF, respectively.

Comparison of separation and detection techniques for human growth hormone releasing factor (hGRF) and the products derived from deamidation

Separation of the deamidation products, Asp8 Leu27 hGRF(1-32)NH2 (MH+ = 3654) and isoAsp8 Leu27 hGRF(1-32)NH2 (MH+ = 3654), from the parent analogue Leu27 hGRF(1-32)NH2 (MH+ = 3653) was achieved by reversed-phase LC and CE, where the retention order was seen to change from tr isoAsp8 hGRF < tr Asn8 hGRF < tr Asp8 hGRF to tr Asn8 hGRF < tr Asp8 hGRF < tr isoAsp8 hGRF, respectively. Both reversed-phase LC and CE gave adequate separations, limits of detection and standard curves. However, CE was preferred due to shorter analysis time, better separation and a smaller demand for material. Packed capillary LC with ESI-MS was then compared with UV detection. On-line LC-MS was found to offer the most efficient approach to detection and identification of hGRF analogues within a single methodology. Identification of Asn8 hGRF from the isobaric deamidation products was achieved from analysis of the triply charged states, where the species were separated by 0.5 amu. LC-MS separation and identification of degradation products offers a viable alternative to fraction collection and subsequent sequencing or enzymatic identification methods. The method becomes increasingly useful for such cases as trace degradation product identification, minimal sample availability or instability of resulting degradation products.

Identification and quantitation of tetrapeptide deamidation products by mass spectrometry

A method to quantify asparagine (Asn), aspartate (Asp) and isoaspartate (isoAsp) residues in small peptides by fast atom bombardment mass spectrometry (FAB-MS) was developed. Discrimination of isoAsp from Asp residues was accomplished by selective derivatization of isoAsp residues in acetic anhydride, D2O and pyridine. Deuteration occurred at any carbon adjacent to a free alpha-carboxyl group, through a transient oxazalone intermediate, allowing the isoAsp side chain and the C-terminus to incorporate deuterium. Thus, isoAsp-containing peptides incorporate one more deuterium than peptides with Asp and two more than Asn peptides. FAB CID-MS spectra of the Asn tetrapeptide, Thr-Asn-Ser-Tyr, were used to confirm the position of deuteration to the C-terminal residue. FAB and FAB CID-MS spectra demonstrated that the 1 amu shift in mass was not caused by derivatization induced deamidation of the Asn residue. FAB-MS spectra of deuterated peptide standards and mixtures containing deamidation products were obtained over the molecular ion region and deconvoluted using non-deuterated control spectra. Deuterium incorporation values for the Asn, Asp and iosAsp containing peptide standards were 80% mono-deuterated peptide, 95% mono-deuterated peptide and 63% di-deuterated peptide, respectively. IsoAsp to Asp ratios in an unknown mixture were obtained by a least-squares minimization of the difference between the unknown deuterated mixture and the isotopic envelopes from the deuterated standards. The mixture was found to contain 85% isoAsp peptide by FAB-MS, which agreed well with 81% isoAsp peptide when assayed by reversed-phase LC.

Ground state saturated population distribution of OH in an acetylene-air flame measured by two optical double resonance pump-probe approaches

Two optical double-resonance pump-probe techniques were used to determine the ground-state rotational population distributions of OH in an acetylene-air flame when a saturating laser beam is tuned to the Q(1)4 transition of the (0, 0) Sigma-II band. The saturated absorption technique is based on the detection of absorption by a probe laser under conditions of saturation with a pump laser and no saturation. In the fluorescence technique, a probe laser is scanned through the (1, 0) band, while a saturating pump laser, tuned to the (0, 0) band, is on or off. We found that approximately 15% of the total population of the ground state was transferred to the excited state. Perturbation of the rotational population distribution was greater for rotational levels close to the directly excited laser-coupled level. The rotational energy transfer rate in the ground state was somewhat greater than in the excited state. The assumption of the balanced cross-rate model was verified as a means of determining the absoslute OH number density with adequate accuracy.