A procedure to optimize scale-up for the primary drying phase of lyophilization.

This article describes a procedure to facilitate scale-up for the primary drying phase of lyophilization using a combination of empirical testing and numerical modeling. Freeze dry microscopy is used to determine the temperature at which lyophile collapse occurs. A laboratory scale freeze-dryer equipped with manometric temperature measurement is utilized to characterize the formulation-dependent mass transfer resistance of the lyophile and develop an optimized laboratory scale primary drying phase of the freeze-drying cycle. Characterization of heat transfer at both lab and pilot scales has been ascertained from data collected during a lyophilization cycle involving surrogate material. Using the empirically derived mass transfer resistance and heat transfer data, a semi-empirical computational heat and mass transfer model originally developed by Mascarenhas et al. (Mascarenhas et al., 1997, Comput Methods Appl Mech Eng 148: 105-124) is demonstrated to provide predictive primary drying data at both the laboratory and pilot scale. Excellent agreement in both the sublimation interface temperature profiles and the time for completion of primary drying is obtained between the experimental cycles and the numerical model at both the laboratory and pilot scales. Further, the computational model predicts the optimum operational settings of the pilot scale lyophilizer, thus the procedure discussed here offers the potential to both reduce the time necessary to develop commercial freeze-drying cycles by eliminating experimentation and to minimize consumption of valuable pharmacologically active materials during process development.

Effect of lipid-derived second messengers on electrophysiological taste responses in the gerbil.

Integrated chorda tympani (CT) recordings were made to salty, sour, sweet, bitter, and glutamate tastants before and after a 4-min application of modulators of lipid-derived second messenger systems. The modulators included two membrane-permeable analogues of DAG, 1-oleoyl-2-acetyl glycerol (OAG) and dioctanoyl glycerol (DiC8); thapsigargin, which releases Ca++ from intracellular stores; ionomycin, a calcium ionophore; lanthanum chloride, an inorganic calcium channel blocker; nifedipine, a dihydropyridine calcium channel blocker; quinacrine diHCl, a phospholipase A2 antagonist; melittin, a phospholipase A2 agonist; and indomethacin, which decreases the release of prostaglandins by inhibiting the enzyme cyclo-oxygenase. The main findings were: OAG (125 microM) and DiC8 (100 microM) blocked the responses of several bitter compounds while enhancing the taste response to several sweeteners. Lanthanum chloride blocked all responses, which may be due to the fact that it blocks tight junctions. Quinacrine (1 mM) suppressed several bitter responses while enhancing the response to several sweeteners. The enhancement of sweet taste responses by DAG analogues suggests that there is cross-talk between the adenylate cyclase system and one (or more) pathways involving lipid-derived second messengers in taste cells.

Taste perception of bitter compounds in young and elderly persons: relation to lipophilicity of bitter compounds.

Threshold and suprathreshold sensitivities to 13 bitter compounds were determined for 16 young adults (mean age = 27.4 years) and 18 elderly persons (mean age = 81.3 years). Half of the subjects in each age group were tasters of the bitter compound phenylthiocarbamide (PTC) and half were nontasters. Both detection and recognition thresholds, determined by a forced-choice ascending detection method, were elevated in older subjects; there were no significant differences in threshold values between tasters and nontasters of PTC. A strong relationship between bitter threshold values and the logarithm of the octanol/water partition coefficient was found for both young and elderly subjects. For young subjects, suprathreshold bitterness ratings were more intense for tasters of PTC compared with nontasters. Significant losses in suprathreshold sensitivity to bitter tastants with age were also found. However, unlike threshold sensitivity, no relationship was found between suprathreshold bitter taste intensity and lipophilicity.

Physicochemical characterization of the various solid forms of carbovir, an antiviral nucleoside.

Carbovir, which exhibits promising in-vitro activity against HIV, is shown to exist in five forms: I, II, III, IV, and V. Forms I-III and V were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hot-stage microscopy (HSM), Karl Fischer titrimetry (KFT), powder X-ray diffraction (PXD), intrinsic dissolution rate (IDR) studies, heat of solution measurements (SC), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, and water uptake at various relative humidities (water activities). Form IV could not be characterized fully, as it is stable only over a narrow temperature range (267-275 degrees C) which is immediately followed by melting and decomposition. With increasing temperature in DSC, forms I and V transformed successively to form III (195 degrees C), then to form II (220 degrees C), and then to form IV (275 degrees C). The PXD patterns, FTIR spectroscopy, IDR, and SC showed significant differences between these polymorphs. For each of the forms I, II, and III, there exists a critical value of relative humidity above which absorption of water proceeds steeply, leading to the formation of form V, which is more heavily hydrated than any of the other forms. Forms I and V each showed a two-step weight loss in TGA (24-120 degrees C), suggesting the presence of water molecules with two different binding energies probably corresponding to two different locations in the crystal lattice; HSM confirmed the dehydration.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulators of the adenylate cyclase system can alter electrophysiological taste responses in gerbil.

The adenylate cyclase system has been implicated in taste transduction. The purpose of this study was to determine whether application of modulators of the adenylate cyclase system to the tongue alter taste responses. Integrated chorda tympani (CT) recordings were made in gerbils to bitter, sweet, salty, sour, and glutamate tastants before and after a 4-min application of four types of modulators of the adenylate cyclase system. The four types of modulators tested were: a) NaF, a compound that promotes dissociation of GTP binding protein; b) forskolin, a powerful stimulant of adenylate cyclase; c) 8-bromoadenosine 3' :5'-cyclic monophosphate sodium salt (8BrcAMP) and N6,2'-O-dibutyryl-adenosine 3' :5'-cyclic monophosphate sodium salt (DBcAMP), two membrane permeable forms of cAMP; and d) 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine dihydrochloride (H-7) and N-(2-[methylamino]ethyl)-5-isoquinolinesulfonamide dihydrochloride) (H-8), which are protein kinase inhibitors. The tast compounds tested were: NaCl (30 mM), monosodium glutamate-MSG (50 mM), sucrose (30 mM), HCl (5 mM and 10 mM), KCl (300 mM), quinine HCl (30 mM), MgCl2 (30 mM), erythromycin (0.7 mM and 1 mM), HCl (5 mM and 10 mM), and urea (2 M). The main findings were as follows. NaF (20 mM) significantly inhibited responses to bitter compounds up to 35% and enhanced the response to sucrose by 30%. NaCl (20 mM), used as a control for NaF, inhibited most responses up to 78% with no enhancement of sucrose as seen with NaF. 8BrcAMP (1.16 mM) reduced the responses to bitter-tasting quinine HCl, MgCl2, and erythromycin but not to urea.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of glass transition temperatures in freeze concentrated solutions of non-electrolytes by electrical thermal analysis.

The electrical resistance (R) of frozen aqueous solutions was measured as a function of temperature in order to determine whether this technique can be applied for determination of glass transition temperatures of maximally freeze concentrated solutions (Tg') of non-electrolytes which do not crystallize during freezing. Electrical thermal analysis (ETA) thermograms of frozen solutions containing the solute alone show a gradual change in slope over the temperature range of interest, with no inflection point which corresponds to Tg'. However, addition of low levels (about 0.1%) of electrolyte changes the shape of the thermogram into a biexponential function where the intersection of the two linear portions of the log (R) vs. T plot corresponds to the glass transition region. The total change in log (R) over the temperature range studied increases as the ionic radius of the reporter ion increases. The sharpest inflection points in the log (R) vs T curves, and the best correlation with DSC results, were obtained with ammonium salts. Tg' values measured by ETA were compared with values measured by DSC. DSC thermograms of solutes with and without electrolyte (0.1%) show that the electrolyte decreases Tg' by about 0.5 to 1.0 degrees C. However, Tg' values measured by ETA are somewhat higher than those measured by DSC, and difference between the two methods seems to increase as Tg' decreases. Tg' as measured by ETA is less heating rate dependent than DSC analysis, and ETA is a more sensitive method than DSC at low solute concentrations and at low heating rates.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of sweeteners on bitter taste in young and elderly subjects.

The purpose of this study was to quantify the degree of reduction in perceived bitterness by sweeteners at both threshold and suprathreshold concentrations of bitter compounds. Detection and recognition thresholds were determined for six bitter compounds (caffeine, denatonium benzoate, magnesium chloride, quinine hydrochloride, sucrose octaacetate, and urea) in the absence and presence of several suprathreshold concentrations of five sweeteners. The sweeteners were: sucrose, aspartame, sodium saccharin, mannitol, and sorbitol. Polycose was also tested along with the sweeteners. The degree to which bitter thresholds were affected by the addition of sweeteners was dependent on the chemical classification of the sweeteners and their concentrations. In general, the natural sweeteners, sucrose, mannitol, and sorbitol, were more effective than the noncaloric sweeteners, aspartame and sodium saccharin, in elevating the detection and recognition thresholds of the bitter compounds. A sweetness intensity approximating that of 6% sucrose (0.175 M sucrose) or greater was required to elevate thresholds. For elderly subjects, sweeteners did not significantly elevate thresholds for denatonium benzoate and sucrose octaacetate. The degree to which sorbitol and sucrose can decrease the perceived bitterness intensity of suprathreshold concentrations of the six bitter compounds was also determined. The concentrations of sweeteners and bitter compounds were selected to be of moderate to high subjective intensity. The levels of sweeteners used in the mixtures were: sucrose (none, 0.946 M, and 2.13 M) and sorbitol (none, 2.1 M, and 3.68 M). Both sweeteners significantly reduced the bitterness ratings of almost every concentration of the six bitter compounds. The greatest reductions in bitterness were 87.0% for 0.192 microM denatonium benzoate mixed with 2.13 M sucrose and 84.7% for 1.8 M urea mixed with 3.68 M sorbitol.

Protein purification process engineering. Freeze drying: A practical overview.

Freeze drying provides a valuable tool to the formulation scientist by permitting dehydration of heat-sensitive drugs and biologicals at low temperature. The final product is quickly and easily reconstituted, and the process is compatible with aseptic operations. Freezing is a critical step, since the microstructure established by the freezing process usually represents the microstructure of the dried product. The product must be frozen to a low enough temperature to be completely solidified. If the solute crystallizes during freezing, this temperature is the eutectic temperature. If the solute remains substantially amorphous with freezing, the relevant temperature is the collapse temperature. Understanding the physical form of the solute--crystalline or amorphous--after freezing can be important from the standpoint of drying characteristics, appearance of the final product, and even product stability during storage. Supercooling is a significant factor in freezing of formulations intended for freeze drying--prior to both primary and secondary (eutectic) crystallization. The driving force for freeze drying is the difference in vapor pressure of ice between the sublimation zone and the condenser. Because the vapor pressure of ice increases sharply with increased product temperature, it is important from the standpoint of process efficiency to maintain product temperature as high as possible during primary drying without damaging the product. The upper limit of product temperature during primary drying again depends on the physical form of the solute. Exceeding either the eutectic temperature (crystalline solute) or the collapse temperature (amorphous solute) results in loss of the desirable properties of a freeze dried product. Freeze drying is a coupled heat and mass transfer process, where either heat transfer or mass transfer may be rate limiting with respect to the overall drying rate. Heat transfer is often the rate-limiting transfer operation because of the high heat of sublimation of ice and the inefficiency of heat transfer. Conduction is the primary mechanism of heat transfer, as opposed to convection or thermal radiation. The rate-limiting resistance to heat transfer is usually the interfacial, or contact, resistance caused by poor contact between materials--the heated shelf, metal trays, and the bottom surface of glass vials. Since the thermal conductivity of a gas is directly proportional to pressure in the free molecular flow regime, the chamber pressure during primary drying is an important determinant of the overall heat transfer rate. As a result, the drying rate for a heat transfer-limited process increases sharply with chamber pressure up to a pressure where free molecular flow conditions no longer apply.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of a phase transition in a frozen solution.

The increased use of lyophilization in the preservation of biologicals, foods, and pharmaceuticals has revealed problems of discoloration, meltback, puffing, and collapse, which can result in unacceptable products. Literature reports suggest that some of these problems are related to phase transitions occurring in the frozen solutions. This paper describes the crystallization of mannitol and cefazolin sodium in frozen solution. Data were collected using calorimetric methods in both an isothermal and non-isothermal mode. Analysis of cefazolin sodium data, using the Johnson-Mehl-Avrami equation, indicated that there was a mechanism of heterogeneous nucleation of a two dimensional growth habit. Mannitol data indicated that the controlling crystallization mechanism was not constant over the temperature range studied.