Advances in endocrinology are happening quickly, and four areas in particular merit our attention to ensure we are keeping
up-to-date and providing the best care for our patients.
David S. Bruyette, DVM, DACVIM
ALTERNATIVES TO PZI VET
In April, IDEXX Pharmaceuticals announced the discontinuation of PZI Vet (a porcine-and bovine-source protamine zinc insulin).
Consider these options for switching cats from PZI Vet to other insulin preparations:
- Vetsulin (Intervet), a porcine insulin zinc suspension originally approved for use in dogs, is now also approved for use in
cats and is a U-40 insulin, like PZI Vet. I would start by giving the same dose of Vetsulin as you had been giving with PZI
Vet. In addition, dosing guidelines can be found at
http://www.vetsulin.com/PDF/Vetsulin_Dear_Doctor_Letter.pdf. If patients had been receiving PZI Vet once a day, I would prepare the owners for twice-a-day administration because it
is unlikely that once-a-day dosing of Vetsulin will be sufficient. Reassess glycemic control seven to 10 days after the switch.
- Humulin N (Eli Lilly), a human recombinant NPH, is a U-100 insulin. I would start by administering the same dose of Humulin
N as you had been giving with PZI Vet. As with Vetsulin, if the cat had been receiving PZI Vet once a day, I would prepare
the owner for twice-a-day administration. Reassess glycemic control seven to 10 days after the switch.
- Glargine (Lantus—insulin glargine [rDNA origin] injection—Sanofi Aventis), a long-acting insulin analogue, is also a U-100
product. Although it is a viable option when switching from PZI Vet, I think it is unlikely to have any advantage over Vetsulin
or Humulin N. The cost would also be greater. And in my opinion, while glargine is an excellent choice for treating diabetes
in newly diagnosed cases, you won't see the same high remission response rate in cats with long-standing diabetes.
I do not recommend nor would I begin using compounded PZI insulin because clinical impression suggests that these preparations
may result in poor or erratic glycemic control.
As noted above, glargine is a good insulin option, particularly in cats in which diabetes has been newly diagnosed. The commercially
available form of glargine, Lantus, is a human insulin analogue produced by recombinant DNA technology. The production organism
is a nonpathogenic laboratory strain of Escherichia coli. Each milliliter contains 100 IU (3.6378 mg) insulin glargine.
At a pH of 4, Lantus is designed to be completely soluble at a neutral pH. After injection into the subcutaneous tissue in
people, the solution is neutralized, and microprecipitates form and slowly release small amounts of insulin glargine, resulting
in a relatively constant concentration over 24 hours.
A study presented at the ACVIM Forum in 2005 showed that eight of eight diabetic cats treated with glargine twice daily and
fed a low-carbohydrate, high-protein diet were in diabetic remission within four months, and six of seven were still in remission
at one year.1 My clinical experience has been similar, with more than 50% of cats newly diagnosed with diabetes experiencing remission
within two to four months when receiving glargine and a low-carbohydrate, high-protein diet. The recommended starting dose
is 0.5 U/kg twice a day if the initial fasting blood glucose concentration is > 360 mg/dl and 0.25 U/kg twice a day if the
initial fasting blood glucose concentration is < 360 mg/dl.1
If you decide to use glargine in your diabetic feline patients, I suggest these tips:
- Do not dilute or mix glargine because it will affect pH.
- Keep glargine refrigerated. Once opened, the vial has a shelf life of four weeks at room temperature. I would discard any
remaining glargine after eight weeks of refrigeration, pending further clinical data.
- After starting glargine, keep in mind that many cats have negligible decreases in blood glucose concentrations in the first
three days: Do not increase the insulin dose. By day 10 after beginning glargine, most cats have good glycemic control.
- Obtain samples at time 0 (preprandial and preinsulin) and then every four hours for 12 hours or until the nadir (lowest glucose
concentration) is observed. It is rare that a curve will need to be performed for a full 24 hours.
- Recheck the cat and perform a glucose curve one, two, three, and four weeks later and then as required.
- Use the preinsulin and nadir blood glucose concentrations obtained with in-house analyzers or at-home blood glucose monitors
in conjunction with the cat's clinical signs to adjust the insulin dose and to monitor for diabetes remission.
Canine hyperadrenocorticism has been most commonly treated with the adrenolytic drug mitotane (o,p'-DDD). However, mitotane
has several side effects and is associated with a high frequency of relapses.
Trilostane is a synthetic, orally active steroid analogue. It can act as a competitive inhibitor of the 3-beta-hydroxysteroid
dehydrogenase enzyme system and, thus, inhibit the synthesis of several steroids, including cortisol and aldosterone. This
blockade is reversible and likely dose-related.
In the United Kingdom, trilostane is licensed to treat canine hyperadrenocorticism. Dechra Pharmaceuticals is pursuing U.S.
FDA approval of trilostane for this condition.
The efficacy and safety of trilostane in treating canine pituitary-dependent hyperadrenocorticism (PDH) were evaluated in
a multicenter European study.2 Seventy-eight dogs with confirmed PDH were treated with trilostane for up to three years. Trilostane appeared to be well-tolerated
by almost all the dogs. Two dogs developed signs and biochemical evidence of hypoadrenocorticism. One of these dogs recovered
with therapy; the other dog died despite withdrawing trilostane and administering therapy. Two additional dogs died within
one week of starting trilostane, but a direct link with the trilostane therapy could not be established in either case.
The study revealed that trilostane was nearly as effective as mitotane in resolving signs of hyperadrenocorticism such as
polyuria, polydipsia, polyphagia, and skin abnormalities.2 These improvements were maintained as long as the dogs received adequate doses of trilostane. Only eight dogs that were
treated with trilostane for more than two months showed poor control of clinical signs. Mitotane is effective in controlling
clinical signs in about 80% of cases of PDH but has a higher percentage of side effects than trilostane does.3
Trilostane also caused a significant reduction in the mean basal and post-ACTH stimulation cortisol concentrations.2 These improvements were also maintained for the duration of the trial. Thirty-five dogs had at least one dose adjustment
during the treatment period.
The mean survival time of all trilostane-treated dogs was 661 days.2 Direct comparison with mitotane is difficult as 65% of the dogs were still alive at censoring, so the mean survival time
may be higher. In comparison, the mean survival time of mitotane-treated dogs is 810 to 900 days.4,5 A recent study comparing trilostane (administered twice daily) to mitotane (administered by using the nonselective adrenocorticolysis
protocol) demonstrated a significantly longer median survival time in the dogs treated with trilostane (900 days) than in
the dogs treated with mitotane (720 days).5
Canine adrenal-dependent hyperadrenocorticism
Only a few cases of adrenal-dependent hyperadrenocorticism have been treated with trilostane. A reduction in post-ACTH cortisol
concentrations has been demonstrated, and survival times of more than two years have been achieved in some cases.6,7 Because of an insufficient number of documented cases, it is impossible to compare the long-term efficacy of trilostane
with that of mitotane in treating adrenal-dependent hyperadrenocorticism. Trilostane is not cytotoxic, and it is likely to
be inferior to mitotane in preventing and controlling metastatic disease. The value of trilostane for preoperative therapy
before adrenalectomy has not been systematically examined. However, given the data above, trilostane might be an effective
and safer alternative to ketoconazole in this respect.