See also: DPP-4 inhibitors: Clinical data and DPP-4 Loss of function models and DPP-4 and the Cardiovascular System

DPP-4 inhibitors approved for the treatment of type 2 diabetes include Vildagliptin (Galvus) (LAF237) , Sitagliptin (Januvia), Saxagliptin and Linagliptin (Tradjenta);  Alogliptin remains an investigational agents in North America.


DPP-4 Inhibitors-Mechanism of action for glycemic reduction

There continues to be intense interest and debate on the specific DPP-4 substrates important for glucoregulation as outlined in a series of publications: The therapeutic actions of DPP-IV inhibition are not mediated by glucagon-like peptide-1. Diabetologia. 2005 Mar 11; [Epub ahead of print] and Glucagon-like peptide-1 mediates the therapeutic actions of DPP-IV inhibitors. Diabetologia. 2005 Mar 10; [Epub ahead of print]

Although a large number of potential peptide substrates  might contribute to glucose regulation following DPP-4 inhibition, the focus of attention has been on the roles of GLP-1 and GIP. Studies in mice lacking either the GLP-1 or GIP receptors (single incretin receptor knockout mice) have demonstrated that DPP-4 inhibitors continue to lower glucose if only one incretin receptor gene is inactivated. In contrast, mice with inactivation of both incretin receptors (dual incretin receptor knockout-DIRKO) mice exhibit normal body weight and fail to exhibit an improved glycemic response following exogenous administration of GIP or the GLP-1R agonist exendin-4. Plasma glucagon and the hypoglycemic response to exogenous insulin were normal in DIRKO mice. Glycemic excursion was abnormally increased and levels of glucose-stimulated insulin secretion were decreased following oral but not intraperitoneal glucose challenge in DIRKO compared to GIPR-/- or Glp1rR-/- mice. Similarly, glucose-stimulated insulin secretion and the response to forskolin were preserved in perifused DIRKO islets. Although the dipeptidyl peptidase-4 (DPP-4) inhibitors valine pyrrolidide (Val-Pyr), LAF237 and SYR106124 lowered glucose and increased plasma insulin in wildtype and single incretin receptor knockout mice, the glucose lowering actions of DPP-4 inhibitors were eliminated in DIRKO mice as outlined in Double Incretin Receptor Knockout (DIRKO) Mice Reveal an Essential Role for the Enteroinsular Axis in Transducing the Glucoregulatory Actions of DPP-IV Inhibitors Diabetes 2004 53: 1326-1335

As the above studies employed single dose administration of DPP-4 inhibitors, it remained possible that more chronic treatment with DPP-4 inhibitors recruits additional substrates, beyond GLP-1 and GIP, that are important for glucoregulation. Flock and colleagues examined this possibility by treating high fat-fed glucose intolerant mice with continuous vildagliptin in the drinking water for 8 weeks to wild type or double incretin receptor knockout (DIRKO) mice.  Vildagliptin lowered blood glucose, stimulated insulin secretion, and reduced plasma levels of cholesterol only in WT but not in DIRKO mice. Hence, the GIP and GLP-1 receptors appear to represent the dominant signaling pathways for the glucose lowering actions of DPP-4 inhibitors-see The incretin receptors for GLP-1 and GIP are essential for the sustained metabolic actions of vildagliptin in mice. Diabetes. 2007 Aug 23; [Epub ahead of print]

Intriguingly, acute studies of the action of DPP-4 inhibitors in mice through use of sitagliptin demonstrated that DPP-4 inhibition is asociated with rapid enhancement of insulin action in liver with suppression of hepatic glucose production in normal mice, as defined by hyperinsulinemic euglycemic clamp studies together with concomitant somatostatin infusion as outlined in Acute DPP-4 Inhibition rapidly Enhances Insulin-mediated Suppression of Endogenous Glucose Production in Mice. Endocrinology. 2008 Sep 18. [Epub ahead of print]

A major pharmacodynamic action of DPP4 inhibitors is the suppression of glucagon secretion from islet a cells, likely through a GLP-1-somatostatin a-cell axis. This was first demonstrated clinically in a study of vildagliptin action Inhibition of dipeptidyl peptidase-4 reduces glycemia, sustains insulin levels, and reduces glucagon levels in type 2 diabetes J Clin Endocrinol Metab. 2004 May;89(5):2078-84. Intriguingly, this property of suppressing glucagon secretion may be especially valuable in the context of combination therapy with SGLT2 inhibitors, which increase glucagon secretion in diabetic subjects. Adding saxagliptin to dapagliflozin nicely suppressed the elevated plasma glucagon levels seen with dapagliflozin, in association with an additional drop in levels of A1c. See Postprandial Dynamics of Plasma Glucose, Insulin, and Glucagon in Patients with Type 2 Diabetes Treated with Saxagliptin Plus Dapagliflozin Add-On to Metformin Therapy Endocr Pract. 2014 Nov 4:1-29.

What is the optimal degree of DPP-4 inhibition require to treat 2 diabetes? The answer to this question remains unresolved, as the degree of HbA1c reduction in clinical trials appears similar with many DPP-4 inhibitors. The observation that plasma levels of intact GLP-1 are only modestly elevated after administration of DPP-4 inhibitors, whereas the degree of glycemic control achieved can be quite reasonable, raises the question of how useful plasma GLP-1 and plasma DPP-4 activity really are as indicative biomarkers for understanding the glucoregulatory efficacy of DPP-4 inhibitors. Waget and colleagues examined the importance of local intestinal vs. systemic DPP-4 activity for glucoregulation using low dose DPP-4 inhibition in WT and incretin receptor knockout mice. Remarkably, administration of very low doses of sitagliptin to mice that inhibited DPP-4 activity selectively in the intestinal mucosa, but not in the systemic circulation, still achieved significant glucoregulation. Low dose sitagliptin also triggered enhanced firing of the vagal nerve in a GLP-1R-dependent manner, and the glucoregulatory efficacy of low dose sitagliptin was diminished in either Glp1r-/- or Gipr-/- mice. Hence DPP-4 in the gut may be disproportionately important for mediating the glucoregulatory actions of the DPP-4 inhibitors. Physiological and Pharmacological Mechanisms through which the DPP-4 Inhibitor Sitagliptin Regulates Glycemia in Mice Endocrinology. 2011 Jun 14.

Does baseline DPP-4 activity or circulating levels of CD26 correlate with the magnitude of response to therapy with DPP-4 inhibitors? Circulating DPP-4 activity tends to be higher in obese subjects and lower in older subjects. Aso and colleagues studied the response to sitagliptin therapy over 24 weeks in 52 Japanese patients with type 2 diabetes. A significant association between baseline levels of A1c and change in A1c was detected; the higher the baseline levels of CD26, the lower the response to therapy. Serum or plasma DPP-4 activity was not reported in this study, although the authors infer the two measurements are correlated. Serum level of soluble CD26/dipeptidyl peptidase-4 (DPP-4) predicts the response to sitagliptin, a DPP-4 inhibitor, in patients with type 2 diabetes controlled inadequately by metformin and/or sulfonylurea Transl Res. 2012 Jan;159(1):25-31.

DPP-4 Inhibitors and Type 2 Diabetes-Overview and Preclinical data

DPP-4 inhibitors, islet growth, neogenesis, and β-cell survival

As both GLP-1 and GIP stimulate β-cell proliferation and inhibit β-cell death, it seems reasonable to predict that DPP-4 inhibition, leading to increased levels of GIP and GLP-1, might also produce beneficial effects on islet mass and β-cell survival. Experiments by Pospisilik and colleagues using the inhibitor P32/98 demonstrated that diabetic rats exhibited increased weight gain, improved blood glucose and glucose tolerance, and enhanced insulin secretion and pancreatic insulin content. Histological examination of the pancreas demonstrated increased numbers of islets and β-cells, and studies in vitro using INS-1 cells demonstrated that both GIP and GLP-1 suppressed STZ-induced apoptosis. See Dipeptidyl Peptidase IV Inhibitor Treatment Stimulates beta-Cell Survival and Islet Neogenesis in Streptozotocin-Induced Diabetic Rats. Diabetes. 2003 Mar;52(3):741-50

The effects of vildagliptin on islet function were examined in female mice expressing a human islet amyloid polypeptide transgene. Vildagliptin improved glucose tolerance, enhanced GSIS and plasma levels of GLP-1, and increased GSIS from isolated islets. Transgenic hIAPP islets displayed abnormal intra-islet topography with centrally located alpha-cells whereas vildagliptin-treated mice exhibited more normal islet topography. See DPP-4 inhibition improves glucose tolerance and increases insulin and GLP-1 responses to gastric glucose in association with normalized islet topography in mice with beta-cell-specific overexpression of human islet amyloid polypeptide. Regul Pept. 2007 Apr 11; [Epub ahead of print]

DPP-4 inhibition has also been shown to exert beneficial effects on transplanted rodent islet cells. Following administration of sitagliptin to mice treated with streptozotocin, there was no significant improvement noted in blood glucose control. However, sitagliptin treatment improved glycemia and islet graft survival following islet transplantation, as outlined in Inhibition of Dipeptidyl Peptidase IV (DPP-IV) with Sitagliptin (MK0431) Prolongs Islet Graft Survival in Streptozotocin (STZ)-induced Diabetic Mice Diabetes. 2008 Mar 25; [Epub ahead of print]

Maida and colleagues compared the actions of a degradation-resistant GIP analogue, with those of exendin-4 or the DPP-4 inhibitor sitagliptin, in murine models of b-cell apoptosis or regeneration in the context of streptozotocin administration. Sitagliptin had a modest effect on b-cell apoptosis but did increase b-cell mass after chronic administration. See Differential importance of GIP vs. GLP-1 receptor signaling for beta-cell survival in mice. Gastroenterology. 2009 Sep 17. [Epub ahead of print]

Sitagliptin treatment for 10 weeks also improved glucose homeostasis, islet topography, pancreatic insulin content, and b-cell area in high fat fed mice treated with STZ. No evidence for stimulation of b-cell proliferation was detected in sitagliptin-treated mice Inhibition of DPP-4 with sitagliptin improves glycemic control and restores islet cell mass and function in a rodent model of type 2 diabetes Eur J Pharmacol. 2009 Sep 15. [Epub ahead of print]

DPP-4 inhibitors-Anti-diabetic actions in preclinical studies

Multiple DPP-4 inhibitors have been characterized, and they appear to lower blood glucose in diabetic rodents via prolongation of GLP-1 and GIP action. For examples of DPP-4 inhibitor efficacy, see Diabetes 1998;47:1253-8 Improved glucose tolerance in Zucker fatty rats by oral administration of the dipeptidyl peptidase IV inhibitor isoleucine thiazolidide and Diabetologia 1999 19;42:1324-1331 Inhibition of dipeptidyl peptidase IV with NVP-DPP728 increases plasma GLP-1 (7-36 amide) concentrations and improves oral glucose tolerance in obese Zucker rats and Improved glucose tolerance and insulin secretion by inhibition of dipeptidyl peptidase IV in mice Eur J Pharmacol 2000 Sep 15;404(1-2):239-245 and Dipeptidyl peptidase IV inhibition reduces the degradation and clearance of GIP and potentiates its insulinotropic and antihyperglycemic effects in anesthetized pigs. Diabetes. 2001 Jul;50(7):1588-97 and Acute and Chronic Effects of the Incretin Enhancer Vildagliptin in Insulin Resistant Rats. J Pharmacol Exp Ther. 2005 Jul 18; [Epub ahead of print]

The 'long-term' effects of chronic oral DPP-4 inhibitor therapy were examined in mildly diabetic fa/fa Zucker rats. Twice daily oral gavage of P32/98, 20 mg/kg/day had no effect on fasting or random glucose but was associated with decreased body weight gain, yet normal food intake, an improvement in oral glucose tolerance and enhanced early phase insulin release in association with increased basal and insulin-stimulated glucose uptake in soleus muscle strips. These effects are impressive given that sustained inhibition of plasma DPP-4 activity was ~ 60% at best, and was even less during the 23:00 to 8:00 am time periods. Glucose profiling demonstrated a significant reduction in blood glucose from 14:00 to 23:00, in association with reduced levels of circulating insulin. A final OGTT done at the end of the 12 week treatment period demonstrated improved glucose tolerance and increased insulin levels at the early time point, but no change in levels of intact GLP-1 during the final OGTT was detected, albeit after drug washout. In contrast, no significant differences in β-cell area or islet morphology were detected at the end of the treatment period. Paradoxically, a progressive increase in fasting DPP-4 activity was seen over the course of the 12 week experiment in the DPP-4 inhibitor-treated rats. See Long-Term Treatment With the Dipeptidyl Peptidase IV Inhibitor P32/98 Causes Sustained Improvements in Glucose Tolerance, Insulin Sensitivity, Hyperinsulinemia, and beta-Cell Glucose Responsiveness in VDF (fa/fa) Zucker Rats. Diabetes. 2002 Apr;51(4):943-950.

Analysis of insulin sensitivity using euglycemic-hyperinsulinemic clamps in treated VDF rats (12 weeks of treatment with P32/98 demonstrated that animals treated with P32/98 exhibited improved hepatic and peripheral insulin sensitivity. No changes in levels of hepatic PEPCK or circulating glucagon were detected in the P32/98-treated animals, however increased insulin-mediated inhibition of lipolysis was observed in isolated epididymal adipocytes from P32/98-treated rats. See Long-Term Treatment With Dipeptidyl Peptidase IV Inhibitor Improves Hepatic and Peripheral Insulin Sensitivity in the VDF Zucker Rat: A Euglycemic- Hyperinsulinemic Clamp Study. Diabetes. 2002 Sep;51(9):2677-83

Similarly impressive results were obtained in studies using a comparatively long-acting inhibitor FE 999011 also administered twice daily to ZDF rats.  This compound produces long-lasting (at least 12 hours) inhibition of plasma DPP-4 activity following a single oral dose of 10 mg/kg. Twice daily administration of this compound produced sustained improvements in plasma glucose over a 25 day time period, associated with a reduction in food consumption and decreased levels of circulating FFAs and triglyceride. See Chronic Inhibition of Circulating Dipeptidyl Peptidase IV by FE 999011 Delays the Occurrence of Diabetes in Male Zucker Diabetic Fatty Rats. Diabetes. 2002 May;51(5):1461-1469

Administration of NVP DPP728 in the drinking water to high fat fed C57BL/6 mice for 8 weeks increased levels of intact GLP-1, improved glucose tolerance, prevented islet hyperplasia and enhanced islet GLUT-2 expression. Glucose-stimulated insulin secretion was also improved following 8 weeks of inhibitor treatment. See Long-term inhibition of dipeptidyl peptidase IV improves glucose tolerance and preserves islet function in mice. Eur J Endocrinol. 2002 May;146(5):717-727

Given the known mechanism of action of DPP-4 inhibitors, principally potentiation of incretin action, it is not surprising that DPP-4 inhibitors are most effective in mild to moderate diabetes, and less effective in the setting of severe insulin resistance. Nagakura and colleagues tested val-pyr (valine-pyrrolidide) in db/db mice of different ages, using acute administration paradigms. Plasma DPP-4 activity was significantly increased in older db/db mice from 6-24 weeks of age compared to C57BL/6J controls. Although Val-Pyr lowered glucose in younger 6 week old db/db mice, no significant reduction of glycemia was observed in older 23 week old severely hyperglycemic db/db mice, despite significant potentiation of circulating GLP-1 following oral glucose loading. See Enteroinsular axis of db/db mice and efficacy of dipeptidyl peptidase IV inhibition.  Metabolism. 2003 Jan; 52(1) :81-6.  Similarly, vildagliptin alone had only modest anti-hyperglycemic effects in db/db mice, whereas the combination of vildagliptin plus the thiazolidinedione rosiglitazone produced similar efficacy to rosiglitazone alone, but without some of the side effects (weight gain, hemodilution) that occurs with rosiglitazone alone. See Combination of dipeptidylpeptidase IV inhibitor and low dose thiazolidinedione: Preclinical efficacy and safety in db/db mice. Life Sci. 2007 May 1; [Epub ahead of print]

DPP-4 inhibitors have also been combined with a-glucosidase inhibitors in preclinical studies, with an additive lowering of blood glucose and enhanced levels of plasma GLP-1 seen in some acute experiments in high fat fed mice. See Comparison of Efficacies of a Dipeptidyl Peptidase IV Inhibitor and alpha-Glucosidase Inhibitors in Oral Carbohydrate and Meal Tolerance Tests and the Effects of Their Combination in Mice. J Pharmacol Sci. 2007 May;104(1):29-38. Epub 2007 May 8

Complementary evidence for the physiological importance of DPP-4 in glucose control is found the phenotype of the Dpp4-/- knockout mouse and rats with a mutation in the Dpp4 gene. To review the data, see DPP-4: Loss of function models

Anti-diabetic drugs and the regulation of plasma DPP-4 activity

Analysis of the relationship between drug therapy and DPP-4 activity in Zucker diabetic rats demonstrated that both metformin and pioglitizaone but not glyburide decreased plasma DPP-4 activity in vivo but not in vitro as shown in Reduced serum dipeptidyl peptidase-IV after metformin and pioglitazone treatments. Biochem Biophys Res Commun. 2004 Nov 5;324(1):92-7. Whether the same types of effects might be observed in human diabetic subjects treated with these drugs remains unclear.

Intriguingly, plasma levels of DPP-4 activity have been demonstrated to be significantly increased in human subjects with type 2 diabetes, with the extent of hyperglycemia correlating with the elevation in DPP-4 activity as described in Hyperglycaemia increases dipeptidyl peptidase IV activity in diabetes mellitus. Diabetologia. 2005 Jun;48(6):1168-72. and Plasma dipeptidyl peptidase-IV activity in patients with type-2 diabetes mellitus correlates positively with HbAlc levels, but is not acutely affected by food intake. Eur J Endocrinol. 2006 Sep;155(3):485-493

Whereas acute metformin administration increases levels of GLP-1 likely indirectly via enhanced secretion, metformin does not directly inhibit DPP-4 as shown in Metformin effects on dipeptidylpeptidase IV degradation of glucagon-like peptide-1. Biochem Biophys Res Commun. 2002 Mar 15;291(5):1302-8. However, there is little data examining DPP-4 levels in patients with diabetes before and after chronic treatment with metformin. Metformin is associated with reduction of DPP-4 activity in human diabetic subjects, particularly when the drug is given in the fasted state Investigation of the effect of oral metformin on dipeptidylpeptidase-4 (DPP-4) activity in Type 2 diabetes Diabet Med. 2009 Jun;26(6):649-54.

Potential substrates for DPP-4 related to insulin secretion

Administration of PACAP, GIP, GRP, or GLP-1 to mice in the presence of the DPP-4 inhibitor val-pyr resulted in the potentiation of insulin secretion in response to intravenous glucose challenge. The potentiating actions of GRP were blocked by the GLP-1 receptor antagonist exendin (9-39). Whether administration of val-pyr alone potentiates the insulinotropic activities of all four of the endogenous peptides remains unclear. Furthermore, whether PACAP and GRP are true physiologically important endogenous substrates for DPP-4 remains unclear. See Inhibition of dipeptidyl peptidase-4 augments insulin secretion in response to exogenously administered glucagon-like Peptide-1, glucose-dependent insulinotropic polypeptide, pituitary adenylate cyclase-activating polypeptide, and gastrin-releasing Peptide in mice. Endocrinology. 2005 Apr;146(4):2055-9

Experiments using mass spectroscopy identified oxyntomodulin and growth hormone [1-43] fragment as new candidate in vivo DPP-4 substrates. Pituitary adenylate cyclase activating polypeptide [1-38] (PACAP38), a neuropeptide insulin secretagogue, was also determined to be efficiently processed by DPP-4 in vitro.  Animals lacking DPP-4 exhibited a significantly slower clearance of circulating PACAP, with virtually complete suppression of the DPP-4 metabolite, PACAP[3-38]. See The role of dipeptidyl peptidase IV in the cleavage of glucagon family peptides: in vivo metabolism of pituitary adenylate cyclase activating polypeptide [1-38]. J Biol Chem. 2003 Apr 10

Analysis of the interaction between DPP-4 inhibitors and relative affinity for putative substrates has been examined as outlined in  Kinetic study of the processing by dipeptidyl-peptidase IV/CD26 of neuropeptides involved in pancreatic insulin secretion. FEBS Lett. 2001 Nov 2;507(3):327-30.

DPP-4 also hydrolyzes glucagon in vitro to generate glucagon (3-29) and glucagon (5-29). These peptides are weak agonists at the glucagon receptor. The biological importance of DPP-4-mediated glucagon degradation for glucagon action in vivo remains unclear. Although glucagon does appear to be a substrate for this enzyme in vitro, the dominant phenotype observed with DPP-4 inhibitors, or in the DPP-4 knockout mouse, is a lower blood glucose and lower levels of glucagon, consistent with the dominant role for DPP-4 in incretin but not glucagon degradation. Furthermore, increased levels of endogenous glucagon in association with administration of  DPP-4 inhibitors has not been reported. A detailed biochemical characterization of glucagon and DPP-4 activity is found in Dipeptidyl peptidase IV (DPIV/CD26) degradation of glucagon. Characterization of glucagon degradation products and DPIV-resistant analogs. J Biol Chem. 2000 Feb 11;275(6):3827-34 and Metabolism of glucagon by dipeptidyl peptidase IV (CD26). Regul Pept 2001 Jan 12;96(3):133-141


What is the role for related members of the DPP-4 gene family in the control of GLP-1 and GIP degradation?

Scientists have determined the essential role of DPP-4/CD26 as the key mediator of human GLP-1 degradation. Using Adenosine Deaminase-affinity chromatography to remove human CD26 from plasma, it was clearly shown that CD26/DP-IV is the predominant enzyme responsible for GLP-1 degradation. For the details of this informative experiment, see Molecular characterization of dipeptidyl peptidase activity in serum soluble CD26/dipeptidyl peptidase IV is responsible for the release of X-Pro dipeptides. Eur J Biochem. 2000 Sep;267(17):5608-13

Is DPP-4 the only enzyme that contributes to degradation of GLP-1?

Several studies have implicated a role for neutral endopeptidase 24.11 in the endoproteolysis of GLP-1. See Characterization of the processing by human neutral endopeptidase 24.11 of GLP-1(7-36) amide and comparison of the substrate specificity of the enzyme for other glucagon-like peptides. Regul Pept. 1995 Aug 22;58(3):149-56 and Endoproteolysis of glucagon-like peptide (GLP)-1 (7-36) amide by ectopeptidases in RINm5F cells. Peptides. 1997;18(5):625-32. At present, there is little evidence from use of inhibitors or genetic studies to ascertain the relative importance of NEP 24.11 for GLP-1 biology in vivo.

GLP-1 (9-36amide): Agonist, antagonist, or biologically inactive degradation product?

DPP-4 inhibitors prevent or attenuate the normal degradation of GLP-1(7-36) amide to GLP-1(9-36) amide. Several studies have raised the possibility that GLP-1(9-36) amide, which is proportionately more abundant than the full length bioactive peptide, may not simply represent an inert cleavage product, but may function either as an endogenous GLP-1R antagonist or a weak agonist (Glucagon-like peptide-1-(9-36) amide is a major metabolite of glucagon-like peptide-1-(7-36) amide after in vivo administration to dogs, and it acts as an antagonist on the pancreatic receptor. Eur J Pharmacol. 1996 Dec  30;318(2-3):429-35 and The inhibitory effect of glucagon-like peptide-1 (7-36)amide on antral motility is antagonized by its N-terminally truncated primary metabolite GLP-1 (9-36)amide. Peptides. 1998;19(5):877-82 and High potency antagonists of the pancreatic glucagon-like peptide-1 receptor. J Biol Chem. 1997 Aug 22;272(34):21201-6, or as a unique agonist with insulin-independent glucose-lowering properties, as hypothesized in GLP-1-(9-36) amide reduces blood glucose in anesthetized pigs by a mechanism that does not involve insulin secretion. Am J Physiol Endocrinol Metab. 2002 Apr;282(4):E873-9.

The importance of GLP-1(9-36)amide has been assessed in healthy humans, where elegant studies have shown no effect of this peptide on insulin secretion or glucose clearance, as shown in Effects of GLP-1-(7-36)NH(2), GLP-1-(7-37), and GLP-1- (9-36)NH(2) on Intravenous Glucose Tolerance and Glucose-Induced Insulin Secretion in Healthy Humans. J Clin Endocrinol Metab. 2003 Apr;88(4):1772-9

For a summary overview of the biology of GLP-1(9-36)amide, see GLP-1(9-36)amide

Given the large number of substrates cleaved by DPP-4 with diverse effects particularly in the immune and endocrine systems, the long term safety of DPP-4 inhibitors in human studies merits careful consideration and analysis. Furthermore, every one of the DPP-4 inhibitors in clinical development is a unique chemical entity and will exhibit a profile of adverse events specific to that chemical entity which may not be generalizable as a "class effect"