Test Formulation Dependent Variables

Test formulations can be infused by many routes at many sites. The most common route of infusion is intravenous but intrathecal, epidural, subcutaneous, intracerebral ventricular, intraduodenal among others, are also used. The determination of the route of administration depends on many factors related to the test article and formulation. How the test article is absorbed, distributed, metaboltized and excreted play a major role in this determination. Pharmacologic effect as well physical and chemical characteristics of the formulation also may have a direct influence. For the most part, discussions here will focus on administration of intravenous solutions.

As a rule, whenever possible, all solutions intended for IV infusion should be adjusted for both pH and tonicity prior to infusion. Solutions with very high or very low pH’s can cause serious local irritation. Solutions that are very hypotonic or hypertonic may not be hemocompatible and may also cause serious complications.

What are the interval, period and duration of infusion?

There are many veins that can be accessed for intravenous infusion (table 1). The vein that is chosen for the infusion often depends on the period, interval, duration and frequency of dosing. By definition, the period is the length of time the infusion is being administered each day, (seconds to 24 hours). The interval relates to intermittent infusions and is the measure of time between infusions. Frequency is the number of infusions (per unit time) and duration is the length of time between the beginning of the first infusion and the end of the last. Short periods of infusions with relatively long intervals between doses allow the use of peripheral veins. Easy access, and the ability to restrain the animal for short periods, make peripheral veins ideal for infusions up to four hours. Peripheral vessels can be accessed multiple times and sites can be rotated or moved to avoid chronic injury. Peripheral percutaneous catheters may also be secured with adhesive backed tape or sutures to provide temporary anchoring and thereby reduce the number of “sticks”.

In general, when dosing periods are longer, it is advantageous to anchor the catheter more securely in a larger vessel. Anchoring reduces the chance that the animal will dislodge the catheter while unrestrained, allowing the animal more freedom of movement during longer infusion periods. For example, to administer infusions of one to two hours, multiple times a day in large animals, a peripheral vein-catheter can be taped down and accessed, for up to 48 hours, allowing the animal untethered freedom of movement. Over a span of many days it is more reliable to anchor to a larger vessel (i.e.with suture beads) and subcutaneously tunnel to an appropriate exit site. Unfortunately, as described here, chronic anchoring of catheters in larger vessels requires a surgical procedure.

Infusion durations up to 2 weeks, however, can be accomplished utilizing an 18ga, 10cm polyurethane catheter (see Clini-cath page 46) in the jugular vein of a free roaming canine, following a less invasive surgical procedure. This technique is not recommended for longer durations due to the close proximity of the vessel insertion and percutaneous access sites. This close proximity shortens the distance bacteria must travel to gain entry into the bloodstream. The longer the catheter remains in place the better the opportunity bacteria has to travel that distance and cause potentially serious complications. The use of tunneled catheters and subcutaneous ports can greatly increase this distance and better isolate the venous and percutaneous sites from one another.

In rodents, the peripheral vein of choice is the tail-vein. This vein is easily accessed with a needle or tail-vein catheter while the animal is restrained for up to 4 hours. Using a tail vein catheter, tail-cuff, tether and swivel, this access can be maintained up to two weeks without invasive surgery.

Central venous catheters are usually placed in the jugular or femoral veins and tunneled either to the mid-scapular region or the base of the tail. Best results are obtained with jugular cannulations when the tip of the catheter extends close to (not into) the atrium. The turbulence with the catheter tip at this location helps keep it clear of thrombi. However, if the catheter is inserted further, into the heart, it can cause severe cardiac irritation.

Jugular cannulas are easy to place and are well suited for infusion and blood sampling. Although many researchers prefer jugular vein access due to the relative ease of implantation, in my experience, femoral vein catheters are more reliable in long-term applications. Best results with femoral vein catheters are achieved when the catheter is inserted into the posterior vena cava so the tip is between the femoral and renal bifurcations.

Central veins should also be employed when the test article or formulation is potentially caustic or irritating to the vessel. The higher level of dilution provided by larger vessels will help minimize these effects. The reduced size of peripheral veins also contraindicates their use when administering large volumes at high flow rates.

What materials are compatible with the test formulation?

The chemical properties of the test formulation must be compatible with the materials used the components that come in contact with the fluid path. Test formulations can breakdown these materials resulting in malfunctions and exposing your test subjects to unintended test articles. Test articles (especially proteins) can adhere to the inside of these components resulting in lower than intended exposures. The only sure way to find out if your system is compatible is to run the test formulation through it, analyze what comes out and inspect the system afterwards. Components that are negatively affected, can then be substituted with those of alternate material composition. If it appears that the test article is sticking to the catheter or tubing, often prefilling (“charging”) them with the test formulation and storing them (usually at refrigerator temperature) a short time to bind up the available adsorption sites will usually solve this problem.

The most common leachate researchers should be aware of is Di(2-ethylhexyl)phlthalate or DEHP. DEHP is a plasticizer that is typically added to poly vinyl chloride (PVC) medical devices (IV bags, extension lines, catheter tubing….) to increase the flexibility of the polymer. The amount of DEHP that will leach out increases with temperature, lipid content, and duration of contact. A wide-range of adverse effects have been seen in laboratory animals, the most concerning on the development of the male reproductive system. The best way to avoid concerns regarding DEHP is to use polymers other than PVC like polyurethane, polyethylene, silicone or simply a non-DEHP PVC. Fortunately, there are many non-DEHP PVC’s available, in fact all of SAI’s PVC extension lines are non-DEHP.

Material tracking issues, chemical compatibility, Good Manufacturing Practices and reliability are by far the best reasons to use FDA and ISO 10993-1, medically approved for human use components. Medically approved devices must adhere to strict guidelines for quality and maintain extensive records to track all of the components that are used in their manufacture. These records can be queried whenever serious questions arise.

What volume and rate are being delivered?

One of the most controversial questions encountered in infusion related research is: what is the highest volume or rate that can be delivered without adverse effects? The actual question should be: what volume of test article formulation can be delivered such that test article related effects are not masked or exacerbated by the volume administered? There are numerous references that will help guide you to recommended safe volumes and rates of infusion depending on the vehicle used in the test formulation. As a general rule, since low volumes will have less effect than high volumes, low volumes and rates are recommended. Unfortunately, in preclinical research, the highest dosage attainable is often determined by the solubility of the test article and higher volumes must be used. Infusing at very low rates is also a very difficult proposition for practical reasons. Verifying very low flow rates is very difficult and dead volumes can often exceed intended volumes of delivery.

The volume and rate of delivery will have a great impact on pump selection. For the most part, you must choose a pump that can accurately deliver the test volume at the rate or rates required. A quick scan of the specifications listed in the pump section of this manual (pages 60-69) will help you decide which pump or pumps are suitable. Note, the relatively large volumetric ench-top/cage-mountable pumps provide a wide range of volumes and rates of delivery. Syringe pumps also provide a wide range of delivery rates but are limited by the range of standard syringe sizes. When high rates are called for, multiple syringe changes may be necessary. They also tend to be relatively large and/or long to accommodate a fully drawn syringe plunger. For these reasons syringe and volumetric pumps are most suitable for restrained and/or tethered infusion systems.

Rate of delivery become more important when choosing an ambulatory pump since currently there are no ambulatory pumps which can deliver at both the very high and very low ends of the range. Reservoir capacity is also very important to avoid excessive change-outs at higher flow rates and less wasted test solution at very low flow rates.

As mentioned earlier, extremely small catheters may also restrict flow if very high flow rates are chosen. Be sure to check the flow capacity of your catheter of choice when these circumstances are encountered since lowered flow rates or blown catheters may result.