CD4+ Lymphocytes Are an Incomplete Surrogate Marker for Clinical Progression in Persons with Asymptomatic HIV Infection Taking Zidovudine

Identifying a valid surrogate end point for serious clinical events associated with human immunodeficiency virus (HIV) infection, such as the development of acquired immunodeficiency syndrome (AIDS)-defining opportunistic diseases or death, would make it possible for the clinical research of new anti-HIV therapies to be conducted more quickly and possibly with fewer patients [1-3]. However, the use of an end point that is not a true surrogate could undermine research efforts and deprive patients of effective therapies by creating the false impression of the activity of new drugs. The recent clinical trial of antiarrhythmic therapy for asymptomatic or mildly symptomatic ventricular arrhythmia after myocardial infarction illustrates this point well [4]. To determine if a marker is a valid surrogate for progression to a particular clinical end point and whether it will be useful in studies investigating anti-HIV therapies, it is not sufficient to show that a pretreatment marker value is associated with such progression. Rather, we need to show that changes in the marker over time are related to that progression and that the effect of a particular drug on this progression can be explained by its effect on the marker [5]. Recent work by DeGruttola and colleagues [6] has indicated that the absolute number of CD4+ lymphocytes in the peripheral blood (the CD4+ cell count) is not a complete surrogate marker for death in persons with AIDS or advanced AIDS-related complex. Their analyses of data from the original phase II zidovudine (3-azido-3-deoxythymidine, AZT) trial [7] and of data from Protocol 002 of the AIDS Clinical Trials Group [8] showed that only a small portion of zidovudine's effect on survival was statistically related to its effect on the CD4+ cell count. Despite the limitations of these findings, their implications, coupled with the perceived similarity between zidovudine and didanosine (2,3-dideoxyinosine, ddI), appear to have played an important role in the recent Food and Drug Administration (FDA) provisional approval of didanosine for use in persons who cannot tolerate zidovudine or who are not benefiting from zidovudine therapy. Data were available to show that didanosine delayed the decrease in CD4+ cell counts during the first 24 weeks of therapy in persons with symptomatic HIV infection, but the data did not show a direct clinical benefit. This use of a surrogate marker for drug approval marks a precedent in AIDS research and may well pave the way for other drug-approval requests based on the effects on surrogate markers. However, the usefulness of the CD4+ cell count as a surrogate marker for death in persons with AIDS may have little bearing on its merit as a surrogate marker for the development of AIDS in persons with early HIV infection. Surrogate markers are more valuable in early disease populations than in more advanced disease populations. Also, it is not clear whether measurements of CD4+ lymphocytes other than the absolute count correlate better with clinical progression. We assessed the extent to which zidovudine's effect on the development of AIDS is statistically related to its effect on CD4+ lymphocyte levels in persons with asymptomatic HIV infection. Methods Patients We analyzed data from the stratum of Protocol 019 of the AIDS Clinical Trials Group (ACTG 019) that enrolled asymptomatic HIV-infected men and nonpregnant women (ages 18 years or older with 500 or fewer CD4+ cells/mm3) to receive placebo, a 500-mg total daily dose of zidovudine, or a 1500-mg total daily dose of zidovudine. The primary results of this stratum of ACTG 019 have been reported elsewhere [9]. For the purpose of this report, all analyses were based on data available through March 1989, before prophylaxis against Pneumocystis carinii pneumonia, a leading AIDS-defining event, was widely permitted. Furthermore, only patients in whom at least one post-baseline CD4+ cell count had been taken were included in this study. The data from the two zidovudine dose groups were pooled because they were similar with respect to pretreatment characteristics and rates of progression to AIDS [9] and because few such progressions occurred in either arm alone. Laboratory Studies Blood samples were collected at enrollment and at follow-up visits at weeks 8, 16, 32, 48, 64, and 80 by the institutions participating in the trial. The CD4+ cell count was calculated as the product of the leukocyte count, the percentage of lymphocytes in all leukocytes (percent lymphocytes), and the percentage of lymphocytes that were CD4+ cells (CD4+ percent). The leukocyte count and the differential count were measured by standard automated techniques. The CD4+ percent was obtained by flow cytometry. Interlaboratory results were monitored by ACTG quality-control procedures [10]. Statistical Analysis The analyses in this report were based on an intention-to-treat approach. Estimates of distributions of the time-to-progression to AIDS were obtained by using the Kaplan-Meier method [11]. For assessing statistical associations among zidovudine, CD4+ lymphocyte levels, and progression to AIDS, the Cox regression [12] model was fitted with the current CD4+ lymphocyte values, the treatment assignment, or both. The current CD4+ cell count was the most recently measured preceding value. The amount of the zidovudine effect on progression to AIDS that could be explained by its effect on CD4+ lymphocyte levels was assessed by the proportional reduction in the regression coefficient for the treatment assignment after controlling for changes in the CD4+ lymphocyte levels. Separate analyses and analyses stratified by treatment were also made when appropriate. Missing marker values were imputed from trajectories of the most recent values by using a first-order autoregressive model [13] with time-varying autocorrelation coefficients. All reported P values are two sided. Results Effect of Zidovudine on Progression to AIDS and on CD4+ Lymphocytes A total of 1075 patients were assessable: 350 in the placebo group and 725 in the zidovudine groups. After a maximum follow-up of 90 weeks (median, 55 weeks), 44 patients had progressed to AIDS: 24 in the placebo group and 20 in the zidovudine groups (log rank; P = 0.04; Figure 1). The overall progression rates were 6.4 (placebo) and 3.2 (zidovudine) per 100 person-years. Figure 1. AIDS-free probabilities for the placebo group and for the zidovudine groups over time. Lymphocyte values were not available for 3% of patients at week 8, 8% at week 16, 14% at week 32, and 23% to 25% thereafter. A summary of the effects of zidovudine on the CD4+ cell count is shown in Figure 2. The effect of zidovudine on this marker appears to be transient, with an increase in the count for approximately 8 weeks followed by a slow decrease at approximately the same rate as that in patients given placebo. An opposite, adverse effect of zidovudine on the leukocyte count occurred that attenuated the drug's overall effects on the CD4+ cell count. In patients given zidovudine, a decrease occurred in the leukocyte count during the first 16 weeks (average loss of 60 cells/mm3 per week for the first 8 weeks and 30 cells/mm3 per week for the next 8 weeks); the leukocyte count was stable thereafter. The leukocyte count in patients given placebo was relatively stable throughout the observation period. Zidovudine's effect on percent lymphocytes and CD4+ percent was similar to its effect on the CD4+ cell count. Figure 2. Cross-sectional medians of the CD4+ cell counts and the component markers in the placebo group and in the zidovudine groups. Association between Markers and Progression to AIDS Stratified (by treatment) Cox regression showed that the baseline values of the CD4+ cell count, leukocyte count, and CD4+ percent among the component markers and the net CD4+ percent (percentage of CD4+ lymphocytes among all leukocytes) were significantly correlated with the risk for clinical progression (Table 1). For example, a smaller baseline CD4+ cell count (50 cells/mm3) corresponded to a 36% higher risk for progression. When a patient's current markers were used in Cox regression with time-dependent covariates, the CD4+ cell count and all of its components were significantly correlated with the risk for progression (Table 1). Furthermore, after the current marker values had been accounted for, the baseline values were no longer significant. Table 1. Prognostic Value of CD4+ Lymphocyte Levels for Progression to AIDS CD8+ lymphocytes, either the absolute count or the percentage, had no significant value in predicting the progression to AIDS in this HIV-infected but asymptomatic patients (data not shown). Also, no significant effect of zidovudine on CD8+ lymphocyte levels was observed. Effect of Zidovudine on Progression to AIDS that Is Explainable by Changes in CD4+ Lymphocyte Levels If the CD4+ cell count were a complete surrogate marker for progression to AIDS, no residual association between treatment and progression would exist after we controlled for the current CD4+ cell count [5]. However, as seen in Table 2, zidovudine was still significantly correlated with progression after adjustment for the current CD4+ cell count. Additionally, the resulting zidovudine-placebo relative risk was virtually identical to the relative risk of 2.10 obtained if we did not account for the current CD4+ cell count. In contrast, the net CD4+ percent accounted for 37% of zidovudine's effect on progression. In either case, patients given zidovudine had a lower risk for clinical progression than did those given placebo, after we controlled for the current CD4+ cell count. This implies that the relation between the current CD4+ cell count and the risk for progression differed in the two treatment groups. Table 2. Effect of Zidovudine in Delaying Progression to AIDS Adjusted for Its Effect on CD4+ Lymphocyte Levels To quantify this difference, we fitted a time-dependent Cox regression