Parenteral-Tonicity PDF

Parenteral-Tonicity □ Osmosis- movement of solvent molecules across a semi-permeable membrane from an area of high solvent concentration (low solute) to low solvent concentration (high solute) □ In an open system- adding solute lowers the vapour pressure on one side + causes solvent to...

Parenteral-Tonicity □ Osmosis- movement of solvent molecules across a semi-permeable membrane from an area of high solvent concentration (low solute) to low solvent concentration (high solute) □ In an open system- adding solute lowers the vapour pressure on one side + causes solvent to move across the membrane to balance the vapour pressure, raises liquid level on the solution side. As shown in the images below ► Applying pressure to the solution will reverse this movement of solvent molecules ► Osmotic pressure: which is the pressure needed to make the level of fluid on both sides of the membrane the same is the osmotic pressure of the solution ► Open system- changes in volume on either side of membrane not important. ► As the membranes will move ► Applying pressure to the solution will reverse this movement of solvent molecules ► Amount of pressure required to return the system to its original state- known as osmotic pressure of the solution Osmotic pressure: ▪ 'colligative' property- properties of solutions that arise because solute particles disrupt certain physical properties of the solvent. How much pressure being put onto the cell wall Main colligative properties include: → 1 Osmole= 1 mole -Avogadro's N.o of osmotically active substances → Osmolarity- the number of osmotically active substances ( osmol) in 1 L of solution → Osmolality- the number of osmotically active substances (osmol) associated with 1 kg solvent Osmolarity & Tonicity: Tonicity & Osmolality: ◊ Osmolality- property of a particular solution and is independent of any membrane Tonicity & Osmolality: ◊ Osmolality- property of a particular solution and is independent of any membrane ◊ Tonicity- property of a solution in reference to a particular membrane. Tonicity is a measure of osmotic pressure gradient not the osmotic pressure ◊ Tonicity- influenced by solutes that do not cross the membrane Tonicity calculations: ◊ Aim to produce a formulation with the same osmolarity/osmolality as plasma components ◊ Calculate or measure the total osmolarity/ osmolality of the formulation ◊ Drug and all excipient ◊ We can adjust to plasma levels using NaCl or dextrose solution 3 types of methods: ► Concentration method ► Freezing point depression (FPD) method  ◊ Drug and all excipient ◊ We can adjust to plasma levels using NaCl or dextrose solution 3 types of methods: ► Concentration method ► Freezing point depression (FPD) method ► Sodium chloride equivalent method Concentration method: ▪ Assume effects of all components are additive ▪ Add up all components in osmolarity mode ▪ Subtract from 325 mOsmolar (plasma) ▪ Calculate the quantity of NaCl/dextrose required ▪ Mg/L- needs to be converted to G/L (X1000) Freezing point depression (FPD) method: ▪ All colligative properties work on the same basis ▪ Use FPD as a surrogate marker- for osmotic pressure Assume the effects of all components are additive ▪ Freezing point of H20= 0.0 degrees Celsius ▪ Freezing point of plasma and tears = -0.52 degrees Celsius ▪ FPD compared to H20- 0.52 degrees Celsius ▪ Freezing point of 0.9 % w/v NaCl aqueous solution = -0.52 degrees Celsius ▪ Freezing point of 1.0% NaCl aqueous solution= -0.58 degrees Celsius Sodium chloride equivalent method: ► NaCl equivalent- amount of NaCl which has the same osmotic effect Sodium chloride equivalent method: ► NaCl equivalent- amount of NaCl which has the same osmotic effect as 1 g of the drug ► NaCl equivalents are additive NaCl equivalent= 0.2 Take away NaCl equivalence from 0.9 ( standard)- 0.9 is what you are trying to get to → Osmolarity- not the same as tonicity → Both describe solutions- but similarity ends there → Osmolarity is concentration-expressed in units of solute/volume → Measured on a machine- called osmometer and it has units- usually osmoles or milliosmoles per litre → Osmolality- expressed using Kg of water instead of litres → Tonicity- describes what a solution would to a cell's volume at equilibrium, if the cell was placed in the solution → Tonicity- cannot be measured on an osmometer- has no units → Tells us the effect a solution has on a cell. Depends both on the osmolarity of the solution and on whether or not solutes in the solution can enter the cell.