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Background It is unclear whether primary prophylaxis against Pneumocystis carinii pneumonia can be discontinued in patients infected with the human immunodeficiency virus (HIV) who are successfully treated with combination antiretroviral therapy. We prospectively studied the safety of stopping prophylaxis among patients in the Swiss HIV Cohort Study.
Methods Patients were eligible for our study if their CD4 counts had increased to at least 200 cells per cubic millimeter and 14 percent of total lymphocytes while they were receiving combination antiretroviral therapy, with these levels sustained for at least 12 weeks. Prophylaxis was stopped at study entry, and patients were examined every three months thereafter. The development of P. carinii pneumonia was the primary end point, and the development of toxoplasmic encephalitis the secondary end point.
Results Of the 262 patients included in our analysis, 121 (46.2 percent) were positive for IgG antibodies to Toxoplasma gondii at base line. The median CD4 count at study entry was 325 per cubic millimeter (range, 210 to 806); the median nadir CD4 count was 110 per cubic millimeter (range, 0 to 240). During a median follow-up of 11.3 months (range, 3.0 to 18.8), prophylaxis was resumed in nine patients, and two patients died. There were no cases of P. carinii pneumonia or toxoplasmic encephalitis. The one-sided upper 99 percent confidence limit for the incidence of P. carinii pneumonia was 1.9 cases per 100 patient-years (based on 238 patient-years of follow-up). The corresponding figure for toxoplasmic encephalitis was 4.2 per 100 patient-years (based on 110 patient-years of follow-up).
Conclusions Stopping primary prophylaxis against P. carinii pneumonia appears to be safe in HIV-infected patients who are receiving combination antiretroviral treatment and who have had a sustained increase in their CD4 counts to at least 200 cells per cubic millimeter and to at least 14 percent of total lymphocytes.
Combination antiretroviral therapy reduces levels of HIV RNA in plasma and increases CD4 counts in most patients within a few weeks.14 It has also reduced the incidence of new opportunistic infections among patients with advanced HIV infection.15,16 The function of the CD4 cells gained as a result of combination antiretroviral therapy is unclear, however. Several studies have found that the initial increase in CD4 cells during combination antiretroviral therapy is due predominantly to the clonal expansion of memory CD4 cells — a finding that raises the question of whether immunocompetence is truly restored.17,18 Conversely, other groups have shown a normalization of cell-mediated immunity against opportunistic pathogens in functional tests within three to six months after the initiation of therapy.19 In some patients, the T-cell repertoire may reach normal levels three to six months after combination antiretroviral therapy is begun.20
Whether the restoration of the immune response induced by combination antiretroviral therapy is sufficient to allow prophylaxis against P. carinii pneumonia to be discontinued remains unclear. Current guidelines state that the decision about whether to initiate or continue prophylaxis should be made on the basis of the lowest CD4 count ever measured in the patient (nadir count).21 We prospectively studied the safety of discontinuing primary prophylaxis against P. carinii pneumonia in patients receiving combination antiretroviral therapy.
Methods
The Swiss HIV Cohort Study
The Swiss HIV Cohort Study is a prospective cohort study with ongoing enrollment of HIV-infected persons 16 years of age or older. Enrollment in one of seven study centers (in Basel, Bern, Geneva, Lausanne, Lugano, St. Gallen, and Zurich) is independent of the disease stage and the degree of immunosuppression. Information is collected according to standardized criteria on structured forms at registration and at follow-up visits, which are scheduled at six-month intervals. The study design has been described in detail elsewhere.22
Inclusion and Exclusion Criteria
Patients receiving primary prophylaxis against P. carinii pneumonia and combination antiretroviral therapy, who had an increase in their CD4 count to 200 cells per cubic millimeter or higher and to at least 14 percent of total peripheral lymphocytes, and whose counts remained above these values for at least 12 weeks, were eligible for inclusion in our study. The local ethics committees of all seven study centers approved the study, and written informed consent was obtained from participants. The protocol stipulated that prophylaxis would be resumed if the CD4 count fell below at least one of the threshold values on two consecutive measurements. Recruitment started on June 28, 1997, and patients were followed prospectively within the framework of the Swiss HIV Cohort Study, except that CD4 counts were measured every three months. Study patients were instructed to contact the medical center if they had any symptoms suggestive of the presence of P. carinii pneumonia or toxoplasmic encephalitis.
End Points
A definite or presumptive diagnosis of P. carinii pneumonia was the primary study end point. A diagnosis was considered definite if P. carinii was found on microscopical analysis of induced sputum or bronchoalveolar fluid or on histologic examination of a lung specimen. A presumptive diagnosis was made if there was a history of dyspnea on exertion or nonproductive cough in the absence of evidence of bacterial pneumonia, if bronchoalveolar lavage was not performed, and if the patient responded to standard treatment for P. carinii pneumonia.
The secondary end point was toxoplasmic encephalitis. A definite diagnosis was made by histologic examination of brain-biopsy specimens. A presumptive diagnosis was made when the following were present: recent onset of a focal neurologic abnormality consistent with intracranial disease, a reduced level of consciousness, or headache; evidence on computed tomography or magnetic resonance imaging of at least one lesion with a mass effect or positive enhancement with contrast medium; and a response to standard treatment for toxoplasmic encephalitis.
Statistical Analysis
All participants with at least one follow-up evaluation were included in the analysis. The length of follow-up was measured from the date prophylaxis against P. carinii pneumonia was stopped. The closing date for the analysis was January 15, 1999. As of this date, all patients' charts were reviewed and patients were contacted by phone if the treating physician had not seen them within the previous month. If they could not be contacted, data were censored as of the date of the last follow-up visit. Events were assumed to have a Poisson distribution, and exact confidence intervals were calculated for the incidence of end points. All patients were considered to be at risk for P. carinii pneumonia, whereas only patients who were serologically positive for IgG antibodies to T. gondii were considered to be at risk for toxoplasmic encephalitis. We used Stata software (version 5.0, Stata, College Station, Tex.) for statistical analyses.
Historical Comparison Group
We examined the incidence of P. carinii pneumonia and toxoplasmic encephalitis in a historical comparison group of patients enrolled in the Swiss HIV Cohort Study from 1990 through 1994, before highly active antiretroviral therapy became widely available. We stratified patients according to their CD4 count at entry (<200, 200 to 500, or >500 cells per cubic millimeter); we then calculated the incidence of P. carinii pneumonia and toxoplasmic encephalitis in the first year after the date of enrollment. Patients in whom P. carinii pneumonia or toxoplasmic encephalitis was diagnosed within the first month after enrollment were excluded from the analysis, because in these patients, enrollment in the Swiss HIV Cohort Study was probably triggered by these events.
Results
Characteristics of the Patients
A total of 396 patients fulfilled the entry criteria, according to the study data base. As of January 15, 1999, 279 (70.5 percent) of them had been enrolled. Seventeen (6.1 percent) were excluded from the present analysis, for the following reasons: 1 resumed prophylaxis because his CD4 count fell below the threshold value before the first follow-up visit, 1 resumed prophylaxis before the first follow-up visit after an episode of bacterial pneumonia, and 15 had not completed their first follow-up visit at the time the data were analyzed. This analysis therefore includes 262 (93.9 percent) of the 279 patients, of whom 242 (92.4 percent) were followed until the closing date of January 15, 1999.
Data on 20 patients were censored before the closing date of the study, for the following reasons: 9 could not be reached by telephone, 2 died (1 had sudden death and 1 died of hepatocellular carcinoma); and 9 resumed prophylaxis because the CD4 count fell below the threshold value on a single measurement (4 patients) or two measurements (3 patients), because of recurrent episodes of respiratory tract infection that responded to aminopenicillin (1 patient), or for personal reasons (1 patient).
The characteristics of the study participants are shown in Table 1. The median age was 37 years, and two thirds of the patients were male. Sixty-three patients (24.0 percent) were in clinical stage C, according to the criteria of the Centers for Disease Control and Prevention (CDC),23 and 113 (43.1 percent) had had a clinical condition known to be associated with an elevated risk of P. carinii pneumonia (recurrent thrush, candida esophagitis, weight loss, or fever).5 One hundred twenty-one (46.2 percent) were positive for IgG antibodies to T. gondii and thus at risk for toxoplasmic encephalitis. The median CD4 count at study entry was 325 cells per cubic millimeter, and the median nadir (lowest-ever-measured) value was 110 cells per cubic millimeter (Figure 1). Close to 90 percent of the patients were taking combination antiretroviral therapy that included at least one protease inhibitor. Only one patient with a history of disseminated Mycobacterium avium complex infection was receiving long-term macrolide therapy.
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Incidence of Events during Follow-Up
Data on incidence are summarized in Table 2. No diagnosis of P. carinii pneumonia or toxoplasmic encephalitis was recorded during follow-up. The incidence of each was therefore zero. The upper 99 percent Poisson confidence limits for the incidence were 1.93 per 100 patient-years for P. carinii pneumonia and 4.20 per 100 patient-years for toxoplasmic encephalitis. None of the patients had an AIDS-defining illness during follow-up. Four patients had an event that defines CDC stage B: one patient had oral candidiasis; one patient, thrombocytopenia; and two patients, cervical intraepithelial dysplasia.
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Analyses of the historical comparison group were based on 3099 patients with a median age of 31 years, of whom 865 (27.9 percent) were female. The distribution according to risk factors for transmission was as follows: 1275 (41.1 percent) had a history of injection-drug use, 988 (31.9 percent) were men who had had sex with men, 756 (24.4 percent) were classified as having heterosexually acquired infection, and 80 (2.6 percent) had been infected through other or unknown routes. Calculations of incidence were based on 3099 patients and 2875 patient-years of follow-up for P. carinii pneumonia and 1089 patients with a positive IgG antibody test for T. gondii and 1004 patient-years of follow-up for toxoplasmic encephalitis. Data on incidence are shown in Table 3 according to the CD4 cell count at study entry.
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We studied the incidence of P. carinii pneumonia and toxoplasmic encephalitis in patients in whom primary prophylaxis against P. carinii pneumonia was discontinued after their CD4 counts increased to at least 200 cells per cubic millimeter and 14 percent of the total lymphocyte count. We included the percentage of CD4 cells in addition to the absolute CD4 count in our criteria because the percentage of CD4 lymphocytes can be measured with greater reliability than the absolute number25 and the value of 14 percent predicts an absolute count of 200 cells per cubic millimeter in a validated model.26 CD4 counts were required to be above the defined threshold for at least 12 weeks before prophylaxis was stopped so as to achieve a reasonable margin of safety in determining that there had been a sustained increase. We could thus be confident that prophylaxis was being stopped only in patients with CD4 cell counts at levels that are known to confer protection against P. carinii pneumonia and toxoplasmosis in HIV-infected patients who are not treated with combination antiretroviral therapy.
In this population, the incidence of P. carinii pneumonia during the first year after prophylaxis was discontinued was zero, with an upper 99 percent confidence limit of less than 2 cases per 100 patient-years. The number of patients at risk for toxoplasmic encephalitis was lower, so that the estimate of the incidence of this condition is less precise, but the rate in the first year was almost certainly below 5 cases per 100 patient-years.
The estimated risk of P. carinii pneumonia or toxoplasmic encephalitis in our patients was similar to the risk among patients with CD4 counts below 200 cells per cubic millimeter who continue to take trimethoprim–sulfamethoxazole in the setting of a controlled clinical trial,7 and it was smaller than the risk of P. carinii pneumonia in patients who receive inhaled pentamidine or dapsone, with or without pyrimethamine, in clinical trials.6,9,10,27
Our results suggest that combination antiretroviral therapy induces a clinically significant restoration of immunity against P. carinii. Our study subjects were at substantial risk for P. carinii pneumonia before the increase in their CD4 counts. Indeed, half of our patients had CD4 counts below 110 per cubic millimeter before they began combination antiretroviral therapy. This value is identical to the median CD4 count in a previous trial conducted within the framework of the Swiss HIV Cohort Study, in which the incidence of P. carinii pneumonia was 27 per 100 patient-years in the absence of combination antiretroviral therapy and of prophylaxis against P. carinii pneumonia.6 Clearly, many cases of P. carinii pneumonia would have been expected in the present study population had immune reconstitution not been successful.
A historical comparison with patients whose CD4 cell counts never dropped below 200 cells per cubic millimeter and who were not treated with combination antiretroviral therapy provides further evidence of a genuine restoration of immune function in our study group. The median CD4 count at entry in our study was similar to the CD4 count among patients enrolled with CD4 counts of 200 to 500 cells per cubic millimeter during 1990 to 1994, before protease inhibitors became widely available. A comparison of estimates of incidence and confidence intervals for this historical group with the results of our present study group indicates that the estimates are similar.
Our results are in line with those of several ongoing studies that were recently presented in abstracts and preliminary reports.28 Schneider and colleagues observed no events after primary or secondary prophylaxis was stopped, but their study was small and their data did not exclude the possibility of a substantial risk of P. carinii pneumonia.29 We did not study the risk of stopping secondary prophylaxis. Interestingly, preliminary results are also encouraging in studies in which anticytomegalovirus treatment was stopped in patients with cytomegalovirus retinitis who had a good response to highly active antiretroviral therapy.30,31 Stopping secondary prophylaxis against P. carinii pneumonia or toxoplasmic encephalitis may well be safe in such patients, but further data are needed before this approach can be recommended.
The median follow-up in this study was more than 11 months, and more than 100 patients were followed for at least 1 year. However, our study cannot exclude with certainty the possibility of an increase in the risk of P. carinii pneumonia and toxoplasmic encephalitis one year after prophylaxis stopped. Immune function increases with time after the successful initiation of combination antiretroviral therapy.17 Moreover, the incidence of opportunistic infections remains high during the first months of treatment but declines thereafter.32 An increase in the risk of P. carinii pneumonia is therefore unlikely if CD4 counts are maintained at safe levels. If the failure of therapy leads to a decline in CD4 counts to levels below 200 cells per cubic millimeter, therefore, prophylaxis should be reintroduced. We restarted prophylaxis for this reason in only seven patients.
This was an uncontrolled, observational study. The possibility of bias due to the selection of patients at low risk must be considered, but such bias is unlikely. The Swiss HIV Cohort Study includes about 70 percent of all patients with advanced disease in the country.33 A large proportion of the eligible patients were included in our analysis, and those who were not included had characteristics similar to those of the study patients. We considered performing a randomized, controlled trial but concluded that it would not be possible to recruit the large number of patients required to show equivalence between the group in which prophylaxis was stopped and that in which it was continued.
In conclusion, we have shown that it is safe to stop primary prophylaxis against P. carinii pneumonia in patients who have responded to combination antiretroviral therapy with an increase in the CD4 count to at least 200 cells per cubic millimeter and 14 percent of total peripheral lymphocytes that has been sustained for at least 12 weeks. The study participants in this large community-based cohort included men and women at all clinical stages of HIV disease, who had acquired HIV infection through both sexual and parenteral routes. The results should therefore be applicable to patients seen in daily clinical practice, if our entry and follow-up criteria are followed.
Supported by a grant (3600.010.1) from the Swiss Federal Office of Public Health.
We are indebted first of all to our patients, who gave their consent for enrollment in this study knowing that there might be a risk of acquiring opportunistic infections; to the physicians and study nurses of the Swiss HIV Cohort Study centers and the general practitioners who were members of the StopCox Study Team, including Dominique Anwar and Anne Meynard, Geneva; Alexa Krautheim, Verena Werder, and Jacqueline Voggensperger, Basel; Pierre Alexandre Bart, Lausanne; Milo Huber, Daniel Geiser, Christine Schneider, Beatrice Merk, Daniel Oertle, and René Jaccard, Zurich; Antonio Carota, Lorenzo Magenta, Gianluca Vanini, and Marina Russotti, Lugano; Andreas Frank and Claudia Inauen, St. Gallen; and Anne Iten, Bern; and to Anna Christen, who organized the collection of the study data, and Nicola Low, who made useful comments on earlier drafts of the manuscript.
Source Information
From the HIV-Sprechstunde, Inselspital Bern, Bern, Switzerland (H.F., R.M.); the Medical Research Council Health Services Research Collaboration, Department of Social Medicine, University of Bristol, Bristol, United Kingdom (M.E.); the Abteilung für Infektionskrankheiten und Spitalhygiene, Universitätsspital Zürich, Zurich, Switzerland (M.O., M.F.); the Ambulatorio Malattie Infettive, Ospedale Regionale di Lugano, Lugano, Switzerland (E.B.); the Division Maladies Infectieuses, Hôpital Cantonal Universitaire Genève, Geneva, Switzerland (B.H.); the Medizinische Universitätspoliklinik, Kantonsspital Basel, Basel, Switzerland (M.B.); the Division Maladies Infectieuses, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland (A.T.); the Klinik A Innere Medizin, Kantonsspital St. Gallen, St. Gallen, Switzerland (P.L.V.); and the Coordination and Data Center, Swiss HIV Cohort Study, Lausanne, Switzerland (M.R.).
Address reprint requests to Dr. Furrer at HIV-Sprechstunde, Inselspital, CH-3010 Bern, Switzerland.
References
The members of the Swiss HIV Cohort Study are as follows: M. Battegay (cochair of the Scientific Board), E. Bernasconi, P. Bürgisser, M. Egger, P. Erb, W. Fierz, M. Flepp (chair of the Clinics Group), P. Francioli (president of the study), H. Furrer, P. Grob, B. Hirschel (cochair of the Scientific Board), B. Ledergerber, R. Malinverni, L. Matter (chair of the Laboratories Group), A. Meynard, M. Opravil, F. Paccaud, G. Pantaleo, L. Perrin, W. Pichler, J.-C. Piffaretti, M. Rickenbach (head of the data center), P. Sudre, J. Schupbach, A. Telenti, P. Vernazza, and R. Weber.
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