COX 2 inhibitors and nephrotoxicity

Introduction

Over the years, study of renal physiology has tuned and fine-tuned our knowledge on various pathways and mechanisms involved in the functioning of the kidneys. As newer mediators, molecules and receptors are discovered, new theories evolve; some substantiate and some others disprove old notions; only a few promise new avenues for treatment strategies. But not all get translated into useful therapeutic breakthroughs. Nowhere is this phenomenon well exemplified than in the relationship between prostaglandins (PG) and the kidney.

Mechanism of NSAID action

The various inflammatory mediators, Prostaglandins, Thromboxanes and Leucotreines are derived from phospholipids in the cell membrane via the cyclo-oxygenase and lipo-oxygenase pathways.(Figure 1). NSAIDs, as a group, derive their anti-inflammatory capability by inhibiting the cyclo-oxygenase (COX) pathway and hence the synthesis of prostaglandins. Many physiological actions other than inflammation such as maintenance of gastric mucosal integrity and modulation of renal microvascular hemodynamics, renin release, and tubular salt and water reabsorption are also mediated by the prostaglandins. Positive role of prostaglandins in maintaining the GFR in the face of adverse circumstances has been known to us for sometime. Hence, nephrotoxicity due to NSAIDs is rather inevitable.

Figure 1

Aspirin had been known for over 100 years, well ahead of our knowledge about role of prostaglandins in renal function^1,2,3,4, and its analgesic, anti-inflammatory, antithrombotic actions were recognized at various times. By 1938, its gastritis producing nature was well recognized and was distressing to the physicians. In the 1960s, non-aspirin NSAIDs surfaced but they failed to be "safer aspirins". It was getting clear by the 1970s that inhibition of prostaglandin synthesis via a cyclo-oxygenase (COX) enzyme was central to both the therapeutic and toxic effects of aspirin and non-aspirin NSAIDs. In the 1980s, the clinical consequences of nephrotoxicity, induced by NSAIDs, were abundantly documented.^5,6. NSAIDs were recognized to induce edema, hypertension, acute renal failure, and, rarely, the nephrotic syndrome.

 

Cyclo-oxygenase is twosome…COX 1 and COX 2!

To the delight of many, in the 1990s, it was recognized that there were two enzymes COX 1 and COX 2 involved in the cyclo-oxygenase pathway and there arose the possibility of dissociating the good from the bad effects of NSAIDs.^7,8(Figure 2)

Figure 2

Two related isoforms of the COX enzyme have been described^9,10 - COX-1 (PGHS-1) and COX-2 (PGHS-2). They possess 60 percent homology in those amino acid sequences apparently conserved for catalysis of arachidonic acid.^11-15 The most important differences between the two isoforms are the regulation and expression of the enzymes in various tissues:

• COX-1 is expressed in most tissues, ‘constitutively’, but variably. It is described as a "housekeeping" enzyme, regulating normal cellular processes (such as gastric cytoprotection, vascular homeostasis, platelet aggregation, and kidney function), and is stimulated by hormones or growth factors.

• COX-2 is usually undetectable in most tissues; its expression is increased during states of inflammation or, experimentally, in response to mitogenic stimuli – ‘inducible’. As an example, growth factors, phorbol esters, and interleukin-1 stimulate the expression of COX-2 in fibroblasts, while endotoxin serves the same function in monocytes/macrophages.^10,16
• · But now we know that COX-2 is also 'constitutively' expressed in the brain, kidney, bone, and probably in the female reproductive system.¹⁷

Need for Selective COX 2 inhibitors

The differences in the effectiveness with which a particular NSAID inhibits an isoform of cyclooxygenase may affect both its activity and toxicity. It has been proposed that the perfect NSAID would inhibit the inducible COX-2 isoform (thereby decreasing inflammation) without having any effect on the constitutive COX l isoform (thereby minimizing toxicity). Such an agent would maximize effectiveness, without inducing toxicity, particularly gastroduodenal erosions and nephrotoxicity.

Most traditional NSAIDs are nonselective inhibitors of both COX 1 and COX 2. Nimesulide, a selective COX-2 inhibitor, celecoxib, and rofecoxib, two highly selective COX-2 inhibitors, have been in use for sometime now. They have at least a 200 to 300 fold selectivity for inhibition of COX-2 over COX 1. These agents provide analgesia comparable to the nonspecific NSAIDs among patients with rheumatoid arthritis and osteoarthritis and claimed to have several unique benefits, particularly a marked reduction in gastroduodenal toxicity (since COX 1 is involved in gastric cytoprotection) and renal safety. In addition, COX 2 inhibitors have been found to reduce the risk of colonic malignancy. Thus far highly selective COX-2 inhibitors have been extensively used in clinical practice and studies, conferring important benefits with regard to gastrointestinal side effects. But renal safety of these compounds has not been the focus of interest in the available large clinical trials.³⁷

Are COX 2 inhibitors nephrotoxic?

Reports of renal toxicity due to selective COX 2 inhibitors nimesulide, celecoxib, and rofecoxib, are appearing at regular intervals, notwithstanding the initial claims. Nimesulide, a selective COX 2 inhibitor, induced acute renal failure have been known since 1988. In a randomized controlled trial of rofecoxib in elderly persons receiving low salt diet, GFR reduction was as much as caused by non selective NSAID, indomethacin.³⁶

The report by Perazella and Eras¹⁸ cautions us about the renal safety of the highly selective COX 2 inhibitors. Celecoxib and rofecoxib induced transient deterioration of renal function in three patients who were from 63 to 73 years of age.¹⁹ Notably, these patients were chronically ill with multi-system diseases, including hypertension, arteriosclerotic cardiovascular disease, heart failure, diabetes mellitus, and chronic renal failure. More and more literature ^39,40,41 on the renal toxicity of COX 2 inhibitors is accumulating, suggesting a pattern of nephrotoxicity similar to traditional non-steroidal anti-inflammatory drugs. COX 2 inhibitors have also been shown to have adverse effect on blood pressure control in stable hypertensive patients.⁴² These disturbing reports call for rethinking and renewed look at the renal physiology especially with respect to COX 2, Prostaglandins and Angiotensin II, to get more insight.

Role of A II, Prostaglandins and COX-2 in renal physiology

Angiotensin II(A II) is the most important effector of body sodium and blood pressure maintenance. Control of renin secretion is the rate-limiting step in the formation of A II. Prostaglandins(PG) as paracrine mediators, are believed to be important factors that can alter rennin secretion.(Figure 3).

A II enhances PG production in vascular smooth muscle, endothelial and mesangial cells. In the intact kidney AII stimulation of PG production occurs mostly in the vascular space and is stimulated by the endothelial COX 1. However it has been recently shown that AII also augments the expression of COX 2 in the vascular smooth muscle cells. Vasodilator PG’s act as buffers against renal vasoconstriction by A II.

The converse relation, namely, PG stimulating A II release is also true. The macula densa mechanism for the control of renin release has been well established and direct evidence exists for PG dependence of low Nacl- stimulation of renin secretion. But all the while COX protein presence had been established by immunohistology only in cells (vascular endothelial cells, epithelial cells of the Bowman’s capsule and the mesangial cells) away from the granular cells, which secrete renin and there was a gap in our understanduing.

COX-2 receptor in Macula densa

The demonstration of COX 2 in the macula densa and the Thick Ascending Limb (TAL), by Harris et al³⁵ was a breakthrough finding, since it immediately suggested a pathway along which the PG’s produced in the Nacl sensing epithelium could interact with the renin producing granular cells. Now it is very clear that renin secretion and renin gene expression induced by a low-salt-diet and frusemide treatment are mediated partially or completely by COX-2-derived renal prostaglandins. Since the control of renin release is also influenced by other factors than the macula densa, the overall effect of COX 2 inhibition on renin release, A II production and the renal function would depend on the clinical setting. Significant reduction in renin- A II secretion and renal function by COX 2 inhibitors can be expected in conditions in which low Nacl by the macula densa is the major contributing cause for the high reninemic state. The use of diuretics, CCF, cirrhosis, nephrotic syndrome and Bartter’s syndrome are best examples of situations of hyperreninemic state in which renin production is mainly driven by TAL/MD COX 2 and is therefore sensitive to COX 2 suppression.

Now going back to the reports of renal toxicity by COX 2 inhibitors, we find that in most instances, there was a setting of high renin state due to diuretics, cirrhosis, CCF, nephritic syndrome, underlying renal failure or old age.

 

COX 2 inhibitors and neonatal renal failure

The other group of startling reports was in the case of neonatal renal failure caused by maternal ingestion of COX 2 inhibitors. First it was perinatal vasoconstrictive reversible renal insufficiency associated with maternal nimesulide use.³⁸ Then came the case reports of irreversible renal failure following maternal ingestion of nimesulide by Balasubramaniam.J.^3,20 and Licia Peruzi et al.⁹ Maturation arrest of the tubules and the presence of immature fetal glomeruli were well documented in the renal biopsy of the neonate.²⁰ (Figures 4,5). Interestingly the maternal ingestion had taken place in the last part of the pregnancy suggesting the role of COX 2 in the renal development and maturation occurring in the last part of pregnancy and the early neonatal period.

 

Figure 4. Simple tufted glomerulus with corona of visceral epithelial cells

 

 

Role of COX 2 in renal development:

Fetal and early postnatal kidneys possess functional COX activity and are a rich source of prostaglandins.^23,24 Studies have localized COX-2 mRNA and immunoreactive protein in the kidneys of normal rats and determined that renal COX-2 expression is highly developmentally regulated.²⁵ During kidney development, immunoreactive COX-2 is first observed in mid-gestation embryonic stages, notably in cells undergoing induction and/or morphogenesis and for the duration of nephrogenesis (through postnatal wk 2 in the rat. The mature intense form of immunoreactive COX-2 appears primarily in functional nephrons as they mature.²⁵ In the postnatal kidney, COX-2 expression is relatively low at birth, increases in the first two postnatal weeks, and gradually declines to low levels in normal adult rats.²⁵

This expression pattern of COX-2 in the developing kidney is of interest because of the evidence that COX metabolites play important functional and developmental roles in the fetal kidney. There also is evidence that COX metabolites may mediate normal renal development. Chronic administration of indomethacin to pregnant Rhesus monkeys led to renal hypoplasia in the neonates, with kidney mass reduced by 15% compared with control animals.²⁹ The observed defect was specific for the kidney, because in the treated animals, development of other organs was not affected, except for hepatic hypertrophy.

Targeted disruption of murine COX-2 has indicated an important role for this enzyme in renal development.^32,33. At maturity in homozygous COX-2 null mice, the kidneys are small, with fewer developed nephrons than in wild-type kidneys. Undeveloped mesenchymal tissue, immature glomeruli, and dysplastic tubules were present in the outer cortex. No apparent developmental or functional abnormalities have been described in mice with targeted disruption of COX-1.³⁴ Of interest, maternal administration of a selective COX-2 inhibitor to wild-type mice and rats during the fetal and/or perinatal period led to renal lesions similar to that observed in the homozygous COX-2 null mice.²⁶

Chronic use of COX inhibitors during human pregnancy has also been related to fetal renal maldevelopment; kidneys from infants who came to term or died in the early postnatal period had few differentiated proximal tubules in the inner cortex and crowding of the glomeruli.^30,31 The outer cortex was more severely affected, with evidence of poorly differentiated glomeruli, undifferentiated tubule epithelia, and tubular dilation.

The above hypothesis and the elegant experimental evidences on the role of COX 2 in the renal development have been proved right by the reports of neonatal renal failure following maternal ingestion of selective COX 2 inhibitors during the last stages of pregnancy.^20,21 In humans, an increased incidence of oligohydramnios has been observed in women who consumed significant amounts of aspirin, non-selective COX inhibitors or selectiver COX 2 inhibitors during the third trimester of pregnancy.^20,21,28 Because the fetal urine is the source of a significant amount of the amniotic fluid, these studies suggested that inhibition of COX led to the suppression of fetal renal function. Most physicians are careful in using/avoiding drugs only during the early pregnancy for the fear of developmental problems to the fetus. This impressive array of experimental and clinical evidences should put us all on guard as irreversible renal failure can result in the newborn unlike the acute renal failure in adults due to COX 2 inhibitors.

Other roles of COX 2:

Renal COX-2 is up-regulated in a variety of experimental nephritis and ablation models and in human lupus nephritis, and COX-2 inhibitors were beneficial in some (but not all) studies.⁴³ Methotrexate³⁸ and corticosteroids may be acting through COX 2 inhibition when used for rheumatic diseases. These issues clearly need more study.

Conclusion :

 In summary,

• COX-2 mRNA and immunoreactive protein are ‘constitutively’ expressed at high levels in restricted locations in the mammalian kidney, the macula densa and surrounding TALH, and the medullary interstitial cells.
• Regulation of expression during development and in a variety of physiologic and pathophysiologic conditions indicates potentially important roles for COX-2 metabolites in glomerulogenesis, regulation of renal hemodynamics, and the renin-angiotensin system.
• COX 2 inhibitors have been found to be nephrotoxic particularly during nephrogenesis (during last part of pregnancy and early neonatal period) and in clinical settings (salt depleted state, diuretic use, CCF, cirrhosis, nephrotic syndrome and elderly ) of hyper-reninemia secondary to low Nacl delivery to TAL.

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