Question: What colorless gas has historically been used to develop therapeutic drugs?
Context: Lisa E. Hensley is the Associate Director of Science at the Office of the Chief Scientist, National Institute of Allergy and Infectious Disease Integrated Research Facility in Frederick, Maryland.  She was previously a civilian microbiologist in the virology division of the United States Army Medical Research Institute of Infectious Diseases (USAMRIID).  Hensley is one of the premier researchers of some of the world’s most dangerous infections, including Ebola hemorrhagic fever, Severe acute respiratory syndrome (SARS), and smallpox.  She has been involved in research uncovering critical mechanisms in the pathogenesis of hemorrhagic fever viruses, and has used those discoveries to develop candidate therapeutic drugs for their treatment. Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).  A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide).  A gas mixture would contain a variety of pure gases much like the air.  What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles.  This separation usually makes a colorless gas invisible to the human observer.  The interaction of gas particles in the presence of electric and gravitational fields are considered negligible as indicated by the constant velocity vectors in the image.  One type of commonly known gas is steam. Perfluorobutane (PFB) is a colorless gas.  It is a simple fluorocarbon with a "n"-butane skeleton and all the hydrogen atoms replaced with fluorine atoms.  It is used as a replacement for Halon 1301 fire extinguishers, as well as the gas component for newer generation microbubble ultrasound contrast agents.  Sonazoid is one such microbubble formulation developed by Amersham Health that uses perfluorobutane for the gas core. In the field of drug discovery, retrometabolic drug design is a strategy for the design of safer drugs using either a soft drug or targeted drug delivery approaches.  The phrase retrometabolic drug design was coined by Nicholas Bodor.  The method is analogous to retrosynthetic analysis where the synthesis of a target molecule is planned backwards.  In retrometabolic drug design, metabolic reaction information of drugs is used to design parent drugs whose metabolism and distribution can be controlled to target and eliminate the drug to increase efficacy and minimize undesirable side effects.  The new drugs thus designed achieve selective organ and/or therapeutic site drug targeting and produce safe therapeutic agents and safe environmental chemicals.  These approaches represent systematic methodologies that thoroughly integrate structure-activity (SAR) and structure-metabolism (SMR) relationships and are aimed at designing safe, locally active compounds with improved therapeutic index (ratio of benefit vs. side effect). The Minor Use Animal Drug Program (or National Research Support Project 7) is the counterpart for animals of the IR-4 Minor Crop Pest Management Program.  The program targets development of therapeutic drugs for minor species, such as small ruminants and aquatic species, plus support for drugs for minor use within major species.  It is carried out in partnership with the Food and Drug Administration’s (FDA) Center for Veterinary Medicine. The Australian Drug Evaluation Committee or ADEC, was a committee that provided independent scientific advice to the Australian Government regarding therapeutic drugs.  The committee was originally formed in 1963 and more recently authorised under the "Therapeutic Goods Act 1989 (Cth)" as part of the Therapeutic Goods Administration (TGA).  In 2010, ADEC was replaced by the Advisory Committee on Prescription Medicines (ACPM) Nitric oxide (nitrogen oxide, nitrogen monoxide) is a molecular, chemical compound with chemical formula of ·NO.  One of several oxides of nitrogen, it is a colorless gas under standard conditions.  Nitric oxide is a free radical—i.e., its bonding structure includes an unpaired electron, represented by the dot (·) on the nitrogen atom—and it is in the class of heteronuclear diatomic molecules that are of historic theoretical interest (for the insights they gave in formulating early modern theories of bonding).  It is a particularly important intermediate in the chemical industry.  In addition, nitric oxide is formed in nitrogen-oxygen mixtures at high-temperature conditions, such as those found in combustion systems including internal combustion piston engines, gas turbines, industrial furnaces, and boilers.  It is also produced naturally by the extremely high air temperatures produced along the path of lightning in thunderstorms. Carbon monoxide-releasing molecules (CO-RMs) are chemical compounds that release controlled amounts of carbon monoxide (CO) to cells and tissues and are being developed as potential therapeutic agents.  Although long recognized as a poison, CO also exhibits beneficial effects in small doses.  These effects include anti-inflammatory activity, vasodilatation, and cardioprotection.  CO is produced in mammals during the degradation of heme by heme oxygenase-1, a redox-sensitive enzyme induced by oxidative stress.  It is this enzymatic reaction that inspired the development of synthetic CO-RMs.  Therapeutic drugs have historically been developed based on the similar activity of small molecules in biological signaling, examples exist in the case of both HS and NO-releasing drugs. 1,1,1,3,3,3-Hexafluoropropane is an organic chemical, an organofluoride.  It is a colorless gas, usually available in the form of a liquid gas.  It is used as a fire suppression agent, a foaming agent, a highly effective refrigerant, a heat transfer medium, a dielectric gas, a sterilant carrier, a polymerization medium, a carrier fluid, a displacement drying agent, a thermodynamic power cycle working fluid, etc. Bioproduction is the production of biologics-based therapeutic drugs including protein-based therapeutics, vaccines, gene therapies as well as cell therapies; drugs so complex they can only be made in living systems or indeed are a living system (cell therapies).  In practice, ‘bioproduction’ has become loosely synonymous with ‘bioprocessing’ as a way to describe the manufacturing process using, cell culture, chromatography, formulation and related analytical testing for large molecule drugs, vaccines and cellular therapies.  Many combinations of reactor types and culture modes are now available for use in bioproduction: e.g., pharming, rocking wave-agitated bag batch, stirred-tank or air-lift fed-batch, and hollow-fiber or spin-filter perfusion.  No single production format is inherently superior; that determination depends on many manufacturing capabilities, requirements, and goals.  New cell lines, concerns about product quality and safety, emerging biosimilars, worldwide demand for vaccines, and cellular medicine drive new innovative solutions in bioproduction.
Answer: Nitric oxide