Major histocompatibility complex polymorphism: dynamics and consequences of parasite-mediated local adaptation in fishes

Parasitism is a common form of life and represents a strong selective pressure for host organisms. In response to this evolutionary pressure, vertebrates have developed genetically coded defences such as the major histocompatibility complex (MHC). Mechanisms of parasite-mediated selection not only maintain outstanding polymorphism in these genes but have also been proposed to further promote host population divergence and ultimately speciation because it can drive evolution of local adaptation in which MHC genes play a crucial role. This review first highlights the dynamics and complexity of parasite-mediated selection in natural systems, which not only depends on dominating parasite strategies and on the taxonomic diversity of the parasite community but also includes the differences in parasite communities between habitats and niches, creating divergent selection on locally adapted populations. Then the different ways in which MHC genes potentially allow vertebrates to respond to these dynamics and to adapt locally are outlined. Finally, it is proposed that varying selection strength in time and space may lead to variation in the strength of precopulatory reproductive isolation which has evolved to maintain local adaptation.

Disentangling the role of MHC-dependent 'good genes' and 'compatible genes' in mate-choice decisions of three-spined sticklebacks Gasterosteus aculeatus under semi-natural conditions

To investigate and disentangle the role of major histocompatibility complex (MHC)-based 'good genes' and 'compatible genes' in mate choice, three-spined sticklebacks Gasterosteus aculeatus with specific MHC IIB genotypes were allowed to reproduce in an outdoor enclosure system. Here, fish were protected from predators but encountered their natural parasites. Mate choice for an intermediate genetic distance between parental MHC genotypes was observed, which would result in intermediate diversity in the offspring, but no mate choice based on good genes was found under the current semi-natural conditions. Investigation of immunological variables revealed that the less-specific innate immune system was more active in individuals with a genetically more divergent MHC allele repertoire. This suggests the need to compensate for an MHC-diminished T-cell repertoire and potentially explains the observed mate choice for intermediate MHC genetic distance. The present findings support a general pattern of mate choice for intermediate MHC diversity (i.e. compatible genes). In addition, the potentially dynamic role of MHC good genes in mate choice under different parasite pressures is discussed in the light of present and previous results.

Comparison of 24-hour blood pressure profiles in patients with hypertension who were switched from amlodipine to nisoldipine

STUDY OBJECTIVE

To compare 24-hour blood pressure control and adverse effects in patients with essential hypertension who were switched from amlodipine to nisoldipine.

DESIGN

Open-label, one-way crossover study.

SETTING

Cardiac clinic and patients' homes.

PATIENTS

Twenty-five patients with stage I or II essential hypertension stabilized with amlodipine for at least 3 months, of whom 21 patients completed the study.

INTERVENTION

All patients underwent 24-hour ambulatory blood pressure monitoring while receiving amlodipine 5 or 10 mg/day. Patients then were switched to nisoldipine 10 mg/day (> or = 65 yrs old) or 20 mg/day (< 65 yrs old) and returned to the clinic at 2-week intervals to assess cuff blood pressure, heart rate, adverse effects, and compliance. No adverse effects were experienced in 15 of the 25 patients. Lower extremity edema was the most commonly reported adverse effect (four patients). Two patients discontinued treatment because of pulmonary edema in one and chest pain in the other. Two patients were lost to follow-up. After a mean of 10.6 weeks, repeat 24-hour ambulatory monitoring was performed to evaluate blood pressure control with nisoldipine. Systolic and diastolic ambulatory results for daytime, nighttime, and total 24 hours were calculated. For amlodipine versus nisoldipine, no significant differences existed in any of the blood pressure parameters (p>0.05) in the 21 patients who completed the study, except for 24-hour diastolic pressure (p<0.05); however, this latter difference was only 2 mm Hg (nisoldipine 77 mm Hg, amlodipine 75 mm Hg).

CONCLUSION

Both amlodipine and nisoldipine have similar 24-hour ambulatory blood pressure profiles. The frequency of lower extremity edema was no different after the switch to nisoldipine than when the patients were taking amlodipine.

Therapeutic change of HMG-CoA reductase inhibitors in patients with coronary artery disease

STUDY OBJECTIVE

To evaluate short-term outcomes when atorvastatin was substituted for pravastatin or simvastatin in patients with coronary artery disease.

DESIGN

Open-label, fixed-dosage, one-way crossover from pravastatin and simvastatin to atorvastatin.

SETTING

University-affiliated hospital and outpatient clinics.

PATIENTS

Eighty patients with coronary artery disease with a minimum baseline low-density lipoprotein (LDL) above 130 mg/dl: 20 were treated with pravastatin 20 mg/day, 20 with pravastatin 40 mg/day, 20 with simvastatin 20 mg/day, and 20 with simvastatin 40 mg/day for a minimum of 6 months, with a prescription refill rate of 80% or greater. Intervention. Before crossover, patients had a fasting lipid profile determined and were questioned about side effects of pravastatin and simvastatin. All patients were switched to atorvastatin 10 mg/day. After 12 weeks of atorvastatin therapy, a repeat fasting lipid profile was obtained and patients were questioned about side effects with the drug.

MEASUREMENTS AND MAIN RESULTS

Baseline demographic and clinical characteristics of the treatment groups were not significantly different with the exception of a lower baseline LDL in patients receiving pravastatin 20 mg/day. Baseline LDL values were as follows: pravastatin 20 mg/day, 158+/-26 mg/dl; pravastatin 40 mg/day, 176+/-22 mg/dl; simvastatin 20 mg/day, 177+/-27 mg/dl; and simvastatin 40 mg/day, 177+/-27 mg/dl. Reductions in LDL after treatment with pravastatin or simvastatin were as follows: pravastatin 20 mg/day, 22%; pravastatin 40 mg/day, 32%; simvastatin 20 mg/day, 33%; and simvastatin 40 mg/day, 38%. Patients achieving LDL goal with initial therapy were as follows: pravastatin 20 mg/day, 5%; pravastatin 40 mg/day, 5%; simvastatin 20 mg/day, 20%; and simvastatin 40 mg/day, 30%. After the switch to atorvastatin 10 mg/day, reductions in LDL were as follows: pravastatin 20 mg/day, 39% (p<0.001); pravastatin 40 mg/day, 38% (p<0.01); simvastatin 20 mg/day, 39% (p=0.04); and simvastatin 40 mg/day, 38% (p=0.83). Patients achieving LDL goals with atorvastatin 10 mg/day were as follows: pravastatin 20 mg/day, 60%; pravastatin 40 mg/day, 30%; simvastatin 20 mg/day, 25%; and simvastatin 40 mg/day, 30%. The frequency of side effects was similar for all three statins. Based on annual average wholesale price, atorvastatin 10 mg/day was more cost-effective than all pravastatin and simvastatin regimens.

CONCLUSIONS

Therapeutic interchange from pravastatin 20 and 40 mg/day and simvastatin 20 mg/day to atorvastatin 10 mg/day was associated with both cost savings and significant reductions in LDL. The change from simvastatin 40 mg/day to atorvastatin 10 mg/day was associated with cost savings and an equivalent reduction in LDL.

Dofetilide: A new antiarrhythmic agent approved for conversion and/or maintenance of atrial fibrillation/atrial flutter

Dofetilide is a new antiarrhythmic agent recently approved for the conversion of and maintenance of sinus rhythm in patients with atrial fibrillation (AF) and atrial flutter (AFl). Dofetilide is a selective class III antiarrhythmic drug which works by selectively blocking the rapid component of the delayed rectifier outward potassium current (I(kr)). Dofetilide has been shown to prolong the effective refractory period which is accompanied by a dose-dependent prolongation of the QT and QTc intervals, with parallel increases in ventricular refractoriness. Approximately 80% of dofetilide is excreted in the urine which requires dose adjustments in renal insufficiency. The elimination half-life is approximately 10 h in patients with normal renal function. The therapeutic blood level range of dofetilide is presently unknown and monitoring of dofetilide blood levels is not available at this time. Clinical trials have shown dofetilide to be superior to flecainide in converting AFl patients to normal sinus rhythm (NSR) (70% vs. 9%; p<0.01). It also was more effective than sotalol in converting AF and AFl patients to NSR (29% vs. 6%; p<0.05) and maintaining these patients in NSR for up to 1 year (p<0.05). Most patients convert to NSR within 24-36 h. Torsade de pointes is the most serious side effect occurring in 0.3-10.5% of patients and is dose related. Other common side effects include headache, chest pain and dizziness. To minimize the risk of induced arrhythmia, patients initiated or reinitiated on dofetilide should be hospitalized for a minimum of 3 days where continuous electrocardiographic monitoring, evaluation of renal function and serum electrolytes and cardiac resuscitation can be provided.

Aggrenox: a fixed-dose combination of aspirin and dipyridamole

OBJECTIVE

To describe the pharmacology, pharmacokinetics, efficacy, and safety of a fixed-dose combination of aspirin and extended-release (ER) dipyridamole indicated for the secondary prevention of stroke.

DATA SOURCES

Published articles and abstracts were identified from a MEDLINE search (1966-December 1999) using the search terms dipyridamole, aspirin, antiplatelet, antiaggregation, and stroke prevention. Pertinent articles written in English were considered for review. Additional articles were identified from the references of retrieved literature.

STUDY SELECTION AND DATA EXTRACTION

Studies including a combination of aspirin/dipyridamole in human subjects were evaluated. Emphasis was placed on randomized, controlled trials.

DATA SYNTHESIS

Aspirin is a platelet inhibitor that works by inhibiting platelet cyclooxygenase, which reduces the production of thromboxane A2. Dipyridamole is a platelet inhibitor that is thought to work in part by inhibiting platelet cyclic-3',5'-adenosine monophosphate and cyclic-3',5'-guanosine monophosphate phosphodiesterase. The active metabolite of aspirin, salicylic acid, is highly bound to plasma protein and has a plasma half-life of two to three hours. Dipyridamole is also highly bound to plasma proteins, and the ER formulation has a plasma half-life of 13 hours. The first European Stroke Prevention Study (ESPS-1) found the combination of aspirin/dipyridamole to be superior to placebo in the prevention of stroke and transient ischemic attack (TIA). The ESPS-1, however, did not include an aspirin-only treatment arm. Therefore, it was unclear whether the combination of aspirin/dipyridamole was superior to aspirin alone. As a result, a second trial was conducted that included treatment arms of aspirin alone, ER dipyridamole alone, combination therapy, and placebo. The combination of aspirin 25 mg plus ER dipyridamole 200 mg twice daily was shown in the ESPS-2 to be significantly better than either agent given individually in preventing stroke and TIAs (p < 0.001).

CONCLUSIONS

The American College of Chest Physicians (ACCP) recommends aspirin 50-325 mg/d to be the initial antiplatelet of choice for the prevention of atherothrombotic cerebral ischemic events. However, with the favorable results of the ESPS-2, it may be appropriate to substitute aspirin/ER dipyridamole for aspirin alone as the drug of choice. This combination appears to have a favorable adverse effect profile. The relative effectiveness of aspirin/ER dipyridamole compared with clopidogrel and ticlopidine has yet to be determined. If alternative antiplatelet therapy is needed, the ACCP recommends clopidogrel rather than ticlopidine because of its lower incidence of adverse effects. The ACCP further states that the combination of aspirin plus dipyridamole may be more effective than clopidogrel; these agents have a similarly favorable adverse effect profile.

Dofetilide, a new class III antiarrhythmic agent

Dofetilide is a new antiarrhythmic agent recently approved for conversion and maintenance of sinus rhythm in patients with atrial fibrillation (AF) and atrial flutter (AFl). It is a class III antiarrhythmic that works by selectively blocking the rapid component of the delayed rectifier outward potassium current. Dofetilide prolongs the effective refractory period in accessory pathways, both anterograde and retrograde. This can be seen on the electrocardiogram through a dose-dependent prolongation of the QT and QTc intervals, with parallel increases in ventricular refractoriness. Approximately 80% of drug is excreted in urine, so dosing must be based on creatinine clearance. The elimination half-life is approximately 10 hours. In clinical trials dofetilide was superior to flecainide in converting patients with AFl to normal sinus rhythm (NSR; 70% vs 9%, p<0.01). It also was more effective than sotalol in converting patients with both AF and AFl to NSR (29% vs 6%, p<0.05) and maintaining them in NSR for up to 1 year. Most patients converted within 24-36 hours. Dofetilide has a favorable risk:benefit profile. Torsades de pointes is the most serious side effect; it occurs in 0.3-10.5% of patients and is dose related. To minimize the risk of induced arrhythmia, patients who start or restart the drug should be hospitalized a minimum of 3 days for creatinine clearance measurements, continuous electrocardiographic monitoring, and cardiac resuscitation, if necessary.