MDR1 – A Generally Safe Drug Could Turn Deadly
Last weekend I attended my cousin’s outdoor wedding in the horse capital of the world, Ocala Florida, and was thrilled to see that they included their Collie in the wedding party. “Barkie” is certainly part of the family, so why not include him in the festivities? As I sat there enjoying the beautiful ceremony to one side and the setting sun to the other, a thought suddenly entered my mind… I wonder if Mark knows about MDR1. My cousin Mark is also a pharmacist and he works in a chain retail outlet dispensing medication for humans.
The following morning I was invited to a Sunday Brunch with the wedding party and during my congratulatory discussions I managed to ask my cousin about “Barkie” and the current heartworm preventatives he is on. I was aware that Mark does not deal enough in veterinary pharmacy and may not have been familiar with this potentially impaired gene that affects many dogs in the herding breeds including collies like “Barkie,” sheepdogs, shepherds and mix breeds containing a herding group. Thankfully, “Barkie’s” veterinarian was familiar with MDR1, had discussed this with Mark’s girlfriend at the time (now his wife) and knew to prescribe a heartworm prevention that is safe for him.
Many dogs mainly in the herding group have an abnormal MDR1 (Multi drug resistant 1) gene. Having this defective gene prevents drugs from being moved out from the cell resulting in high levels of the drug due to accumulation. Toxic levels, sometimes life threatening, may result when certain drugs accumulate in the body. Although this defective gene is present mainly in dogs within the herding group, a test can be performed to determine whether your dog has this defective gene and also differentiate whether there is one defective gene or two defective genes. Dogs with two defective genes are even more susceptible to drug toxicity therefore even more extreme caution should be taken.
The most common drug that is at risk of accumulation in dogs with MDR1 is ivermectin. Ivermectin is in products such as Heartgard and Iverhart and is used for the prevention of heartworm disease. Accumulation of ivermectin may cause life threatening central nervous system effects. Certain cancer drugs may also accumulate, causing very bad adverse reactions. Digoxin is a cardiac drug that may cause very dangerous toxicity to the heart if it accumulates because of the MDR1 gene mutation. Other drugs such as morphine, codeine, and acepromazine may accumulate causing loss of consciousness, central nervous system depression, and respiratory depression possibly leading to coma or death.There are many other drugs that may be dangerous in pets who suffer from this gene mutation. Your veterinarian is the best individual to decide whether your pet is at risk, order the proper laboratory test, and let you know which pet medications need to be avoided or have their dose reduced.
There are many other reasons for a particular dog to be susceptible to drug accumulation besides having an abnormal MDR1 gene. Cell enzymes, disease states, other medication taken are among the many reasons that may also impair excretion. In order to avoid potentially toxic accumulation of a particular drug, it is extremely important to have a good relationship and discuss every concern that you have with your pet’s veterinarian and your pet’s pharmacist. If you have any questions about any medication your pet is taking, the pharmacists at 1800-Pet-Meds are always more than happy to help answer them for you.