Acute respiratory distress syndrome (ARDS) is a life threatening form of lung injury that results from an impairment of gas exchange due to fluid buildup in the lung. Several experimental treatment modalities are under investigation to reduce the mortality in ARDS patients, including “liquid ventilation” with a biocompatible perfluorocarbon compound (PFC). During liquid ventilation, the diseased lung is partially or totally filled with a PFC. Both animal and clinical studies suggest that liquid ventilation can recruit alveolar volume and facilitate gas exchange in the diseased lung. There is also evidence that PFCs can decrease the inflammatory response characteristic for ARDS and less severe forms of acute lung injury.
PFCs used during liquid ventilation are perfluorinated aliphatic compounds (i.e., all hydrogen atoms in the molecule are replaced by fluorine) that have a high solubility for oxygen and carbon dioxide. They are evenly distributed in the lung due to unique properties, such as low surface tension, low viscosity, high spreading coefficients and high density. Their systemic uptake is minimal because of their low solubility in water, biological fats and lipids. The main route of elimination of PFCs is exhalation because of the combined hydrophobic and lipophobic character.
Because of their properties, PFCs have been suggested as vehicles for the administration of pharmacological agents directly to the diseased lung. This approach has several advantages compared to conventional drug delivery approaches, such as intravenous administration. For example, intrapulmonary drug administration is expected to achieve higher drug concentrations in the lung while at the same time reducing systemic uptake of the drug. At the same time, the even distribution of the PFC in the lung would result in a homogenous distribution of the drug. Furthermore, PFCs can deliver drugs to diseased parts of the lung by displacing fluid present in the diseased lung because of their higher density and immiscibility with water.
Unfortunately, PFCs are extremely poor solvents for typical drug molecules. This represents a challenge for using PFCs as vehicles for the pulmonary administration of drugs. A number of approaches have been investigated to overcome this solubility problem and move PFC-based administration of drugs towards clinical application. These approaches include dispersions of aqueous drug solutions or solid drug particles, PFC-soluble prodrugs and reverse (water-in-PFC) emulsions.
Several animal studies have investigated the administration of PFC-based dispersions of solid drugs or aqueous drug solutions to the diseased lung. Early studies relied on “bulk flow turbulent mixing” in the lung to achieve a homogeneous distribution of the drug in the lung. More recently, porous nanoparticles of drugs have been developed to obtain stable dispersions of the drug in the PFC. These novel dispersions allow a more controlled administration of the drug to the lung. The intrapulmonary administration of dispersions of drugs in a PFC typically had a greater effect on the pulmonary response relative to systemic responses. Drugs administered dispersed in a PFC were evenly distributed within the lung, with higher levels of the drug in the lung compared to intravenous administration.
Some pharmacological agents, such as fluorinated anesthetics, are soluble in PFCs. For example, the anesthetic halothane was successfully delivered to hamsters undergoing liquid ventilation. In addition, the solubility of drug molecules in PFCs can be enhanced by covalently attaching a perfluoroalkyl moiety to the parent drug molecule. This can be accomplished by synthesizing a perfluoroalkyl ester of the drug of interest. Subsequently, a PFC-soluble prodrug can be administered to the diseased lung where it is expected to partition into lung tissue and release the parent drug by chemical or biological degradation. Although this approach has not been investigated in vivo, prodrugs of nicotinic acid have been shown to be soluble in PFCs. Furthermore, they can release the parent drug, nicotinic acid, in vitro and increase cellular levels of NAD.
In addition to dispersions and PFC-soluble prodrugs, reverse water-in-PFC (micro-)emulsions have been investigated for their potential to administer typical drug molecules to the diseased lung. The goal of this approach is to dissolve the drug in the aqueous phase of the (micro-)emulsion while retaining desired properties such as high fluidity and high solubility for oxygen and carbon dioxide. Although this approach is straightforward and versatile, only fluorinated dimorpholinophosphates have been reported to form biocompatible reverse water-in-PFC (micro-)emulsions. The emulsions can dissolve clinically relevant concentrations of a broad range of drugs and are stable for extended periods of time.
Although the results from these studies are encouraging, administration of drugs dispersed in PFCs is still far from a clinical application.
Acute respiratory distress syndrome (ARDS) is a life threatening form of lung injury that results from an impairment of gas exchange due to fluid buildup in the lung. Several experimental treatment modalities are under investigation to reduce the mortality in ARDS patients, including “liquid ventilation” with a biocompatible perfluorocarbon compound (PFC). During liquid ventilation, the diseased lung is partially or totally filled with a PFC. Both animal and clinical studies suggest that liquid ventilation can recruit alveolar volume and facilitate gas exchange in the diseased lung. There is also evidence that PFCs can decrease the inflammatory response characteristic for ARDS and less severe forms of acute lung injury.
PFCs used during liquid ventilation are perfluorinated aliphatic compounds (i.e., all hydrogen atoms in the molecule are replaced by fluorine) that have a high solubility for oxygen and carbon dioxide. They are evenly distributed in the lung due to unique properties, such as low surface tension, low viscosity, high spreading coefficients and high density. Their systemic uptake is minimal because of their low solubility in water, biological fats and lipids. The main route of elimination of PFCs is exhalation because of the combined hydrophobic and lipophobic character.
Because of their properties, PFCs have been suggested as vehicles for the administration of pharmacological agents directly to the diseased lung. This approach has several advantages compared to conventional drug delivery approaches, such as intravenous administration. For example, intrapulmonary drug administration is expected to achieve higher drug concentrations in the lung while at the same time reducing systemic uptake of the drug. At the same time, the even distribution of the PFC in the lung would result in a homogenous distribution of the drug. Furthermore, PFCs can deliver drugs to diseased parts of the lung by displacing fluid present in the diseased lung because of their higher density and immiscibility with water.
Unfortunately, PFCs are extremely poor solvents for typical drug molecules. This represents a challenge for using PFCs as vehicles for the pulmonary administration of drugs. A number of approaches have been investigated to overcome this solubility problem and move PFC-based administration of drugs towards clinical application. These approaches include dispersions of aqueous drug solutions or solid drug particles, PFC-soluble prodrugs and reverse (water-in-PFC) emulsions.
Several animal studies have investigated the administration of PFC-based dispersions of solid drugs or aqueous drug solutions to the diseased lung. Early studies relied on “bulk flow turbulent mixing” in the lung to achieve a homogeneous distribution of the drug in the lung. More recently, porous nanoparticles of drugs have been developed to obtain stable dispersions of the drug in the PFC. These novel dispersions allow a more controlled administration of the drug to the lung. The intrapulmonary administration of dispersions of drugs in a PFC typically had a greater effect on the pulmonary response relative to systemic responses. Drugs administered dispersed in a PFC were evenly distributed within the lung, with higher levels of the drug in the lung compared to intravenous administration.
Some pharmacological agents, such as fluorinated anesthetics, are soluble in PFCs. For example, the anesthetic halothane was successfully delivered to hamsters undergoing liquid ventilation. In addition, the solubility of drug molecules in PFCs can be enhanced by covalently attaching a perfluoroalkyl moiety to the parent drug molecule. This can be accomplished by synthesizing a perfluoroalkyl ester of the drug of interest. Subsequently, a PFC-soluble prodrug can be administered to the diseased lung where it is expected to partition into lung tissue and release the parent drug by chemical or biological degradation. Although this approach has not been investigated in vivo, prodrugs of nicotinic acid have been shown to be soluble in PFCs. Furthermore, they can release the parent drug, nicotinic acid, in vitro and increase cellular levels of NAD.
In addition to dispersions and PFC-soluble prodrugs, reverse water-in-PFC (micro-)emulsions have been investigated for their potential to administer typical drug molecules to the diseased lung. The goal of this approach is to dissolve the drug in the aqueous phase of the (micro-)emulsion while retaining desired properties such as high fluidity and high solubility for oxygen and carbon dioxide. Although this approach is straightforward and versatile, only fluorinated dimorpholinophosphates have been reported to form biocompatible reverse water-in-PFC (micro-)emulsions. The emulsions can dissolve clinically relevant concentrations of a broad range of drugs and are stable for extended periods of time.
Although the results from these studies are encouraging, administration of drugs dispersed in PFCs is still far from a clinical application.
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