Using a Coulter Counter to Measure the Quality of Injectable Fat Emulsion

Drug carriers, imaging agents and injectable drugs for clinical diagnostic instruments are in emulsion form. Some examples are brucea javanica oil emulsion injection, vitamin E emulsion injection, nutritional supplement fat emulsion for parenteral nutrition, ultrasonic diagnostic emulsion and coix lacryma-jobi seed oil injection.

As these substances are injected directly into the circulatory system intravenously and get absorbed via micro-circulation, it is very important that non-toxicity and stability is properly controlled based on regulatory specifications. Therefore the precise choice of emulsion particle analyzer is one of the main factors in quality control of these substances.

Function of Fat Emulsion

Fat emulsion is one of the chief types of intravenous emulsions. The raw material for most clinical fat emulsions is soybean oil; however other vegetable oils are also used such as olive oils.

Image credit: Beckman Coulter

Fat emulsions containing purified soybean oil-in-water as the key constituent have the following benefits:

• Non-toxicity
• Solvency
• Easily absorbed by the human body as it is highly dispersed
• Superior nutritional supplement for parenteral nutrition
• Can be used as a carrier for targeted drug delivery
• Can be used as a tracer for in vivo target allocation

The increase in usage of fat emulsion as a carrier for intravenous injection is because it provides excellent and controlled release, and helps increase targeting for drugs that cannot be directly injected intravenously or for non-dissolvable drugs.

Fat emulsions are frequently used in clinical domain as a carrier for the following:

• Dexamethasone
• Neuroleptic drugs
• Propofol

It is also used in the following intravenous injection drugs:

• Diazepam
• Etomidate
• PGE1
• Dexamethasone palmirate
• Fat dissolvable vitamins
• Alprostadil

Several of the injectable emulsions are alterations of Liposyn II, Intralipid, Lipofundin fat emulsions. Research is being done on drugs such as itraconazole, tetrandrine, paclitaxel, garlic oil, amphotericin B, bupivacaine hydrochloride, and tirilazad, to detemine whether they can be carried by fat emulsion via intravenous injection.

Property and Quality Requirement of Fat Emulsion

An emulsion exhibits thermodynamic instability and is heterogeneously dispersed. Its physical destabilization frequently causes flocculation, phase transition, coalescence, separation, and rupture, and spoilage that may impact emulsion particles and several properties of the unit.

Image credit: Beckman Coulter

In order to greatly stabilize an emulsion, the globule size needs to be decreased and the size should be uniformly maintained. Safety to recipients is another key issue that needs attention. The main safety issues with regards to injectable fat emulsions are impairments in plasma clearance in susceptible patients, and the infusion of an unstable emulsion containing great quantities of potentially embolic fat globules.

The human blood capillaries have internal diameters of 6-8 µm. So if an injection has many fat globules measuring more than 5µm, the infused globules could be embedded into the pulmonary capillaries creating a fat embolism syndrome.

Animals research to test the toxicity from the infusion of unstable injectable fat emulsions revealed that there is oxidative stress and tissue damage to the liver when the suggested globule size limits surpass 5µm.

Thus, after several tests, the optimum particle size for fat emulsion is set to be in the range of 0.1-0.5 µm in diameter. These particles resemble human chyle particles in size (< 0.5 µm) and their osmotic pressure (310-350 mmol/L). Additionally, pH (8.0) is almost identical to human blood plasma.

An emulsion is basically examined for the following properties:

• Fat globule size
• Stability constant
• Coalescence rate
• Viscosity
• Layer separation trend.

Industrial Standard

There are very specific standards and designated measurement technology for fat emulsions. In the Chinese Pharmacopoeia 2005, Volume 2, Chapter 6, globules in fat emulsion injection have to, “…use a counter to measure; most particles should have a size about 0.5 µm.

Among particles larger than 0.5 µm, there should be no more than 3 percent of particles larger than 1µm. There should be no particles larger than 5 µm.” However, in USA there are no pharmacopeia standards with respect to the optimal pharmaceutical characteristics of the formulation.

Manufacturing and marketing companies need to strictly follow the standards and provide reports. In case new formulations are created, samples will first need to be submitted to a regulation board such provincial institute for drug control. In China, the Guangdong Institute for Drug Control uses Beckman Coulter’s Coulter Counter Multisizer 3 to examine fat emulsion injection samples.

Why Use a Coulter Counter

The Chinese Pharmacopoeia 2005 stipulates that the tool for quality assurance of fat emulsion has to be a counter not a particle size analyzer. This is to ensure that particles are not larger than 5µm.

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Beckman Coulter’s Coulter Counter is capable of sensitively and precisely locating the existence of any particles more than 5µm in size. In other tools such as laser diffraction and photon correlation spectroscopy, the measurements are affected by color, number, shape, and other optical properties of particles. Beckman Coulter’s Coulter Counter has a better resolution as well.

Many of these technologies do not directly locate particles but derive particle size under theoretical assumptions from measured signals. Several assumptions have to be made such as particles have to be spherical with no multiple scattering. Matrix conversion or functional fitting has to be used for determining relative particle size distribution.

Laser diffraction measurements may miss certain particles if the particle size is more than 5µm but with smaller overall size. This is hazardous to intravenous items. It does not measure particle by particle. Also it will not be able to give the exact number of large particles.

Similarly, in photon correlation spectroscopy, particle size and size distribution are derived from scattering theory from the measured ensemble signal. It is a very low resolution technology that is reserved for use on submicron particles only. Although it is used in formulation and stability study of fat emulsion particle size distribution, it is not capable of measuring absolute particle number or locating a few large particles in the sample.

The Coulter Principle is based on an electrical sensing zone method. It uses the proportionality between the height of the electric pulse generated from passage of the particle via a small aperture and the volume of a particle to directly count it and measure its size. The measurement is not affected by shape, color, and optical features of particles. It can locate particles even in extremely low concentration, which is very beneficial for measuring emulsions with very few particles.

Applications of the Coulter Counter

Beckman Coulter created the Coulter Principle, and has been working in the field of particle characterization for more than fifty years. They have developed over twenty models of Coulter Counter in that time period. The latest one is the Coulter Counter Multisizer 3, which uses digital pulse processing technology. It enables the tool to more precisely record and test every electrical pulse. It also allows the user to monitor sample changes, such as decomposition, flocculation, and cell size change, via pulse analysis.

Image credit: Beckman Coulter

Sino-Swed Pharmaceutical, global leader of medicinal nutrition supplements research, has been relying on the Coulter Counter for product research and quality control needs ever since its establishment in China. Sino-Swed Pharmaceutical knows that the Coulter Counter is a very good choice to directly detect the absolute number of particles, not via model conversion of measured signals. Thus it has always been using Coulter Counter for quality control, even from the previous models, such as TA II, Multisizer IIe, to the latest model Multisizer 3.

Other manufacturers in China that use Coulter Counter for quality control and R&D are listed below:

• Guangzhou Qiaoguang Pharmaceutical
• Mitsubishi Pharma (Guangzhou)
• Chongqing Yaoyou Pharmaceutical
• Zhejiang Kanglaite Pharmaceutical
• Shanghai Xinxing Medicine

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.