ANEMIA IN CHRONIC RENAL FAILURE

Anemia occurs in many patients with chronic renal failure (CRF), and the severity and progression of anemia often correlates with the degree of CRF. However, it is important to keep in mind that anemia may be exacerbated by things other than CRF, such as concurrent diseases or by iatrogenic blood loss associated with repeated sampling of blood for diagnostic tests and monitoring.

Anemia associated with CRF is usually nonregenerative and characterized by normochromic - normocytic red blood cells. However, some patients with CRF may have concurrent iron deficiency, resulting in microcytic--hypochromic red blood cells. Iron deficiency is a relatively common problem in dogs and cats with CRF. In a recent study, serum iron concentrations were below the reference range (transferrin saturations < 20%) in 3 of 6 dogs and 3 of 7 cats with CRF.

Mechanism of Anemia

The mechanism of anemia in patients with CRF is more complex than a relative deficiency of erythropoietin. Although nutritional abnormalities, erythropoietic inhibitor substances in uremic plasma, a shortened red blood cell life span, myelofibrosis, and blood loss may all contribute to anemia, the principal cause of anemia in most patients with CRF is erythropoietin deficiency. Erythropoietin deficiency in CRF is thought to result from an insufficient capacity for new hormone synthesis in response to hypoxia due to decreased renal mass.

Erythropoietin is a hormone secreted on demand by the kidneys in response to intrarenal tissue hypoxia resulting from either decreased oxygen-carrying capacity associated with anemia or decreased oxygen content. It enhances erythropoiesis by stimulating formation of proerythroblasts, as well as promoting hemoglobin synthesis and release of reticulocytes from bone marrow into the circulation. Patients with CRF may have a relative erythropoietin deficiency rather than an absolute deficiency. Plasma levels of erythropoietin may exceed the normal range, however, when compared to nonuremic patients with an equivalent degree of anemia, the plasma erythropoietin concentrations are lower in patients with CRF.

Clinical Signs Associated with Anemia

Anemia in patients with CRF can impact quality of life. Some of the clinical signs associated with anemia include pale mucous membranes, lethargy, weakness, fatigue, cold intolerance, decreased affection, behavioral apathy, and decreased appetite. Many patients will show improvement in their clinical signs when anemia is corrected, and this can happen even in the absence of improvement in their level of azotemia.

Guidelines for Minimizing Anemia

An important, but often overlooked consideration for minimizing anemia in patients with CRF is iatrogenic blood loss. This is particularly a problem in hospitalized cats and small dogs when repeated samples of blood are taken from already anemic patients for diagnostic tests and monitoring. Many times clinicians attribute the anemia to other causes and fail to consider the impact repeated blood sampling can have on small, anemic cats and dogs.

Another important consideration for anemia in patients with CRF is GI blood loss. It is not uncommon for these patients to lack overt GI signs of melena. Indirect evidence of occult GI blood loss is an elevation in BUN/creatinine. Normally BUN should be 10 to 20 times higher than serum creatinine. If GI blood loss is suspected, a trial course of an H₂-receptor antagonist, such as ranitidine or famotidine, in addition to sucralfate, should be considered. Improvements in hematocrit would indicate a positive response to therapy.

Nutritional causes for anemia should also be addressed. Often times patients with CRF have selective appetites and may be at risk for developing iron deficiency. This is particularly a risk factor in patients with GI blood loss or iatrogenic blood sampling anemia. Other less common nutritional causes for anemia in patients with CRF is B vitamin deficiencies, such as deficiencies in riboflavin (vitamin B₂), cobalamin (vitamin B₁₂), folate, niacin, or pyridoxine (vitamin B₆). These are water-soluble vitamins, and patients with CRF may lose excessive quantities of these B vitamins in their urine.

Hormone Replacement Therapy

Erythropoietin therapy is the treatment of choice for non-life threatening anemia in dogs and cats with CRF when hematocrit values fall below 20% and clinical signs are attributable to the anemia. Although recombinant canine and feline erythropoietin have been developed, the only readily commercially available form is recombinant human erythropoietin (r-HuEPO).

Complications of r-HuEPO Therapy in Dogs and Cats

The most commonly recognized side effect of r-HuEPO administration in dogs and cats is the development of refractory anemia and hypoplasia of the erythroid bone marrow associated with the formation of neutralizing anti-r-HuEPO antibodies. Antibody formation is estimated to occur in approximately 50% of dogs and cats that receive r-HuEPO therapy. If antibodies are going to form, they tend to occur within one to 6 months after starting r-HuEPO therapy. What should raise the clinician's level of suspicion that this is occurring is when a dog or cat that has previously responded well to r-HuEPO therapy suddenly has a rapid and progressive decrease in the hematocrit for no apparent reason. If a bone marrow aspirate is performed, erythroid hypoplasia and increased myeloid: erythroid ratios (>8:1) will be observed. If anti-r-HuEPO antibody formation is not recognized early in the course of this problem and r-HuEPO therapy is continued, erythroid hypoplasia and resulting anemia will progress. In some patients, the severity of anemia is worse than before initiation of r-HuEPO treatment. Discontinuing r-HuEPO therapy appears to result in the cessation of antibody formation. However, the rate of recovery from the resulting anemia will depend on the antibody titer and its clearance from the body, as well as the ability of the patient to secrete endogenous (native) erythropoietin.

The relatively high prevalence of anti-r-HuEPO antibody production raises the question of when to initiate r-HuEPO therapy in dogs and cats with CRF. If therapy for anemia is started prior to the patient showing any clinical signs associated with anemia, anti-r-HuEPO antibodies may develop during this time. This deprives the patient of the clinical benefits of this therapy when clinical signs of anemia do develop and the patient could receive the greatest clinical benefit from r-HuEPO therapy. Most patients will not show significant clinical signs of chronic anemia until the hematocrit falls below 20%.

Other less commonly recognized side effects that may be associated with r-HuEPO administration in dogs and cats include polycythemia, systemic hypertension, iron depletion, local reactions at the injection site, allergic reactions, and seizures. Seizures have been observed in human, canine and feline patients receiving r-HuEPO therapy that had no prior history of a seizure disorder. Although seizures are thought to be related to compensatory adaptations to increases in red blood cell mass, seizures are not thought to be directly related to r-HuEPO.

Considerations Prior To Initiating r-HuEPO Therapy

A blood pressure should be evaluated in patients prior to initiating r-HuEPO therapy. If systemic hypertension is present, this should be corrected prior to starting r-HuEPO therapy because r-HuEPO may increase the severity of hypertension. A reduced dosage of r-HuEPO may also need to be given to prevent rapid increases in blood pressure.

The patient's iron status should also be evaluated prior to initiating therapy with r-HuEPO therapy. The erythropoietic response from r-HuEPO requires massive mobilization of iron from tissue stores to support hemoglobin synthesis. Failure to provide the patient with adequate levels of iron can result in a blunted therapeutic response to r-HuEPO therapy or no response at all. In addition to iron deficiency resulting in a poor response to therapy, ignoring the iron deficiency in a patient doesn't prevent the formation of anti-r-HuEPO antibodies.

Iron status can be difficult to assess in dogs and cats. Evaluating the stainable iron content in bone marrow is the most reliable way to assess a patient's iron status, however, this is often impractical to do clinically in most patients. As a result, the most practical way to evaluate the iron status in patients with CRF is with serum iron levels. Serum iron levels can be decreased due to iron deficiency or to chronic inflammatory disease. The distinction is important because anemia of chronic inflammatory disease will not improve with iron supplementation and may even result in iron overload. Serum iron levels may also be normal in some patients with iron deficiency. A supporting test is serum transferrin saturation, which is estimated by dividing serum iron by total iron binding capacity (TIBC). It provides some assessment of the ability of the patient to mobilize iron stores to meet the demands of erythropoiesis.

Initial Dosage of r-HuEPO and Monitoring

In dogs and cats with anemia secondary to CRF, the initial dosage of r-HuEPO is 50-100 units/kg subcutaneously (SQ) three times a week. A target hematocrit in dogs is 37% to 45% and 30% to 40% in cats. The hematocrit should be monitored weekly until the lower end of the target range is reached. At that time, the dosage interval of r-HuEPO can be decreased to once or twice weekly. The hematocrit should continue to be monitored weekly until it has stabilized in the target range for at least 4 weeks. At this time, the hematocrit can be monitored monthly as long as it remains stable.

Generally the target hematocrit is attained within the first 8 to 12 weeks of starting r-HuEPO therapy. However, this is quite variable and depends on the starting dose of r-HuEPO, adequate iron availability, and the starting hematocrit. In some patients, the target hematocrit may be attained before 8 weeks. A poor response to initial therapy should prompt an evaluation for an underlying cause, such as iron deficiency, GI blood loss, and concurrent infectious, inflammatory or neoplastic disease. In addition, if a patient suddenly becomes unresponsive to r-HuEPO therapy, the possibility of anti-r-HuEPO antibody formation, in addition to the above causes, should be considered.

r-HuEPO Therapy and Concurrent Iron Supplementation

Iron supplementation is recommended for all patients receiving r-HuEPO therapy even if the patient does not have iron deficiency. The demand for iron associated with stimulated erythropoiesis is high, and even patients without pre-existing iron deficiency may be unable to keep up with demand for iron.

Iron can be administered orally in the form of ferrous sulfate. Oral iron supplements can be associated with GI upset and diarrhea, however this has been an infrequent occurrence in the authors experience when ferrous sulfate is given. Starting doses of oral ferrous sulfate for dogs is 100-300 mg/day and 50-100 mg/day for cats. Serum iron levels and transferrin saturation should continue to be monitored monthly or bimonthly during iron supplementation.

An alternative route for iron supplementation is intramuscular injections of iron dextran. Sometimes it is necessary to give iron supplementation by this route to restore the iron status in severely iron deficient patients or in patients with GI side effects to the oral route of administration. Failure to respond to oral iron supplementation has been the case in some cats that the author has treated. However, caution should be taken with parenteral administration of iron because there is a small risk of anaphylaxis. Generally, a very small (0.1 ml) test dose of iron dextran is injected first to make sure the patient does not have an immediate anaphylactic reaction. If there is no allergic reaction, the remainder of the dose can be administered 5-10 minutes after the test dose is given. Parenteral administration of iron can also be associated with iron overload and pain at the injection site. Despite the potential drawbacks, parenteral administration of iron is the author's preferred route for iron supplementation in cats. The frequency of iron injections is quite variable, but most cats the author has manage have required iron injections once every 1 to 2 months.

Indications for Discontinuing Iron Supplementation

Erythropoietin therapy should be discontinued if any of the following occur: anti-r-HuEPO antibody formation, systemic hypertension unresponsive to antihypertensive therapy, polycythemia, or suspicion of local or systemic hypersensitivity.

CALCITRIOL THERAPY IN CHRONIC RENAL FAILURE

In CRF, the number of functioning renal tubules progressively decreases, resulting in the loss of renal tubular cells that synthesize calcitriol. In addition to reduced synthesis of calcitriol due to loss of renal tubular cells, high levels of serum phosphorus that accumulate as glomerular filtration rate (GFR) decreases also impairs calcitriol synthesis by inhibiting 1-hydroxylation of 25-hydroxyvitamin D to produce 1,25-dihydroxyvitamin D (calcitriol). Calcitriol is the most biologically active form of vitamin D.

The initial effects of calcitriol deficiency are temporarily halted by a compensatory elevation in parathyroid hormone (PTH) concentrations (renal secondary hyperparathyroidism). The potent activation of calcitriol formation by parathyroid hormone (PTH) is due, in part, to its phosphaturic lowering of renal tubule cell phosphorus levels, thereby partially relieving the inhibition phosphorus has on calcitriol formation. Surprisingly, although high levels of serum phosphorus inhibit calcitriol, lowering serum phosphorus levels below normal does not raise serum calcitriol levels above normal in dogs. In addition, overt hyperphosphatemia is not a prerequisite for phosphorus to have an effect on PTH secretion. High dietary phosphorus intake may accentuate uremia-induced abnormal phosphorous metabolism, causing an increase in parathyroid cell phosphorous concentration, and dietary phosphorus restriction has been shown to decrease PTH secretion without changing serum calcitriol levels. In untreated human patients with mild to moderate CRF (serum creatinine concentration < 3.0 mg/dl), serum phosphorus concentrations correlated directly with PTH, independent of serum calcium and calcitriol levels, and this correlation was present despite the fact that most patients had serum phosphorous concentrations within the normal range.

Renal Secondary Hyperparathyroidism

Renal secondary hyperparathyroidism occurs to some extent in most dogs and cats with CRF. In a recent study in cats with spontaneous CRF, the overall prevalence of renal secondary hyperparathyroidism was 84%. The prevalence of renal secondary hyperparathyroidism in asymptomatic cats with CRF was 47%, and it was 100% in cats with end-stage CRF.

Relative or absolute deficiency of calcitriol is thought to play a central role in the development of renal secondary hyperparathyroidism. Excessive levels of PTH in CRF can have toxic effects on multiple tissues and organs. Some of these toxic effects include:

1.  Renal osteodystrophy

2.  Carbohydrate intolerance by interfering with the response of pancreatic insulin-secreting islet cells to dietary intake of glucose

3.  Inappetence

4.  Defects in fatty acid metabolism

5.  Actions on the brain to cause depression

6.  Slowing of peripheral nerve conduction velocity

7.  Red and white blood cell toxicity contributing to uremic anemia and causing a variety of leukocyte malfunctions including failures of immunologic response

Excess PTH levels may also promote nephrocalcinosis and consequent progressive loss of renal function. Increased PTH causes damage to tissues by interacting with the PTH or PTHrP receptor in target cells to increase levels of intracellular ionic calcium. High cellular calcium is toxic by activating various enzymes that destroy cell membranes, proteins and nucleic acids. Ultimately, accumulation of intracellular calcium disrupts mitochondrial production of ATP, diminishing tissue energy.

Rationale for Calcitriol Therapy

The primary goal of calcitriol therapy is to prevent or correct renal secondary hyperparathyroidism and its adverse effects. Calcitriol decreases PTH levels in blood. Recommendations vary for when in the course of CRF to start calcitriol therapy. However, one potential argument for starting calcitriol supplementation in early CRF is the possibility of minimizing the toxic effects associated with renal secondary hyperparathyroidism. A potential adverse effect of calcitriol supplementation is the development hypercalcemia. Although hypercalcemia reportedly occurs in 30% to 57% of humans treated with calcitriol, hypercalcemia appears to be an uncommon side effect in dogs treated with calcitriol at the recommended doses. However, the potential benefits of calcitriol supplementation in patients with CRF must be balanced with the lack of randomized, controlled clinical trials to support its use. Clinical studies using calcitriol supplementation in both dogs and cats with naturally-occurring CRF are in progress, and results from these studies should help provide a clearer understanding of the pros and cons of its use. It is also important to provide careful medical care.

Recommended Protocol for Calcitriol Therapy in CRF

1. Collect baseline serum creatinine, phosphorus, calcium, BUN and an intact PTH level.

In patients receiving calcitriol therapy, PTH levels should be monitored using intact PTH assays, not carboxy-terminal or mid-molecule specific types of radioimmunoassay.

2. Serum phosphorus concentrations must be reduced to 6.0 mg/dl or lower before initiating calcitriol therapy.

Hyperphosphatemia increases the chances of calcitriol promoting renal mineralization and further renal damage. If serum phosphorus levels are increased despite feeding a phosphorus-restricted diet, use aluminum hydroxide to bind phosphorus from the diet and prevent intestinal absorption. Avoid using calcium carbonate to bind dietary phosphorus because once calcitriol is administered, intestinal absorption of calcium may increase dramatically and result in hypercalcemia.

3. Calcitriol should not be given with meals because it enhances intestinal absorption of calcium and phosphorus.

4. A starting dose of 2.5 to 3.5 ng/kg body weight per day given orally to dogs and cats (note dosage is in nanograms not milligrams).

The optimum maintenance dose for calcitriol must be determined for each patient based on serial evaluation of serum calcium and phosphorus and intact PTH. It is recommended to evaluate serum phosphorus, calcium, creatinine and BUN levels in patients 1 week and 1 month after initial calcitriol therapy, and monthly thereafter if the patient's condition is stable.

The onset of hypercalcemia after initiation of calcitriol therapy is unpredictable because of the dynamic nature of CRF. Persistent monitoring is highly recommended because hypercalcemia may occur after days to months of treatment.

The product of serum calcium and phosphorus concentrations should not exceed 60. The goal is to attain values between 42 and 52.

5. Recommended endpoint of calcitriol therapy is normalization of PTH activity in the absence of hypercalcemia.

If the dose of calcitriol necessary to normalize PTH levels is associated with hypercalcemia, the daily dose may be doubled and given every other day. This approach may be less likely to induce hypercalcemia because the effect of calcitriol on intestinal calcium absorption is related to the duration of exposure of intestinal cells to calcitriol.

6. Calcitriol is commercially available in 0.25 ug and 0.50 ug capsules (1 ug = 1000 ng).

It is often necessary to compound calcitriol for use in our veterinary patients. However, if compounded, it must contain a stabilizing agent to protect its 3 alcoholic groups from oxidation by the polyunsaturated oils used to dilute the calcitriol. It is advisable to use a compounding pharmacy experienced in compounding calcitriol to try to ensure stability of the product.