Human Immunodeficiency Virus (HIV) RNA in Plasma as the Preferred Target for Therapy in Patients with HIV Infection: A Critique

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1 116 AIDS COMMENTARY Human Immunodeficiency Virus (HIV) RNA in Plasma as the Preferred Target for Therapy in Patients with HIV Infection: A Critique W. Jeffrey Fessel HIV Research Unit, Kaiser Permanente, San Francisco, San Francisco, California The measurement of human immunodeficiency virus (HIV) RNA copies in plasma establishes the prognosis for HIV-infected patients and provides a rapid method of assessing the efficacy of a new antiretroviral agent. However, whether or not periodic measurement of plasma HIV RNA will aid physicians in the long-term treatment of infected patients remains to be proven. Dr. Fessel has reviewed the available literature and suggests that the enthusiasm for this technique of managing HIV infection may be premature. Dr. Fessel emphasizes that solid evidence is needed to prove that prolonged suppression of plasma levels of HIV RNA reflects suppression of viral replication in the lymphoreticular tissue and prevents selection of resistant HIV variants. With such evidence, the usefulness of the plasma HIV RNA assay in the treatment of HIV-infected patients will be more firmly established. John P. Phair A stampede of enthusiasm has greeted the introduction of readily available assays of plasma HIV RNA (phiv RNA) levels. This enthusiasm is easy to understand in view of the variability of CD4 + lymphocyte measurements; the fact that CD4 + lymphocytes are incomplete surrogate markers of the risks for progression of HIV infection to AIDS or death [1, 2], the catchy cliche "it's the virus, stupid" [3], and the attraction to both doctor and patient alike of the statement that "the virus is now undetectable in your blood." Considering the paucity of published evidence that created the stampede, it is surprising that no one has discussed the possibilities that the phiv RNA level may be neither the ideal target for therapy nor the improvement over CD4+ lymphocyte counts that its proponents maintain. Levy [4] seems to be a lone voice in suggesting that unless treatments for HIV infection not only reduce plasma viral load but also cellular viral load, the reservoir for renewed production of virus remains and compromises the clinical outcome. Clinical practice often requires that action be taken that goes beyond the data from properly conducted trials. This is acceptable as long as the risks to the patient and the financial costs of such action are proportional to the health problem that is involved, and alternative actions are neither clinically preferable nor less risky or costly. A case in point is the use of the Received 16 October 1996; revised 7 November Reprints or correspondence: Dr. W. Jeffrey Fessel, Director, HIV Research Unit, Kaiser Permanente, San Francisco, 2590 Geary Boulevard, San Francisco, California Clinical Infectious Diseases 1997; 24: by The University of Chicago. All rights reserved /97/ $02.00 phiv RNA level in addition to the CD4 + lymphocyte count as a tool for determining when to initiate or change therapy for patients infected with HIV. The financial costs of widespread phiv RNA testing are extraordinary. Commercial laboratories currently charge at least $120, and often $250, for a single phiv RNA test, whereas the actual cost, including labor, overhead, and reagents, for a lymphocyte phenotyping test for CD4 + and CD8 + lymphocytes is $16. It has been proposed that decisions to either initiate or change therapy should be based on the phiv RNA level regardless of the CD4 + lymphocyte count [5, 6]. As a result of economies of scale, the cost of the test might be reduced by 25% to $90, which is the current at-cost price. The test should be done at least four times yearly for the 1 million patients in the United States; therefore the aggregate, annual, recurring cost will be $360 million ($480 million to $1 billion at the current, inflated prices) an extraordinary cost for a single test. This is a minimal figure, since it is recommended that a test result that suggests the need to change current treatment be confirmed by a repeated test [5]. It might be supposed that a serious proposal that will add between $360 million and $1 billion to the national health care budget, plus an additional $1 billion if patients with CD4 + lymphocyte counts of >500/mm 3 and phiv RNA levels of >10,000 copies/ml were to receive therapy with three drugs (see below), would be supported by strong evidence. It is clear and beyond dispute that a single phiv RNA level yields important prognostic information that is more accurate than that obtained with a single CD4+ lymphocyte count; the phiv RNA level also yields useful information about the shortterm efficacy of antiviral drugs [7]. What is unclear is whether the long-term outcomes of treatment are substantially improved

2 CID 1997;24 (February) HIV RNA in Plasma 117 when treatment is guided by phiv RNA levels in addition to CD4 + lymphocyte counts. It is my purpose to examine the evidence supporting the thesis that the phiv RNA level is preferable to the CD4 + lymphocyte count as a measure of the long-term benefit of treatment. Several pieces of evidence fall short of being totally convincing and will be discussed with respect to four overlapping factors including the assumptions that underlie the thesis, the available evidence that supports these assumptions, some consequences of the recommendations that have been made as a result of these assumptions, and the cost-to-benefit ratio of the application of the thesis. It has been repeatedly stated that the goal of therapy for HIV disease is to reduce HIV in plasma to undetectable levels at all times. However, we risk mistaking repetition for truth. The following assumptions underlie the statement that the goal of therapy for HIV disease is to render the levels of phiv RNA undetectable at all times: (1) short-term decreases in phiv RNA levels can be maintained in the long run without inducing viral resistance or escape; (2) a sufficient number of new, effective drugs to combat the resistant organism will be available, should viral resistance or escape occur; (3) long-term decreases in phiv RNA levels will result in decreases in tissue viral burden; (4) the levels of phiv RNA, as measured, represent free infectious virus rather than a mixture of infectious virus plus virus complexed with its neutralizing antibody; and (5) unacceptably high phiv RNA levels represent a situation that demands urgent action despite acceptable levels of CD4 + lymphocytes. These assumptions seem persuasive on the surface, but when they are examined in detail, they become less convincing. What is the likelihood that short-term suppression of phiv RNA can be maintained in the long run? Data from the prerelease trials of protease inhibitors and nonnucleoside reverse transcriptase inhibitors have shown that impressively high percentages of patients who were treated with these agents had phiv RNA levels that became undetectable by the assays used. In most instances, phiv RNA levels remained suppressed for the duration of follow-up, which has been as long as 36 months for a few patients. Despite this success, whether the suppression will be maintained in all patients for years is uncertain. Viral resistance to every drug used in the past has eventually developed in high percentages of patients; it is argued that the situation has now changed because combination therapy with three or more drugs that include new agents such as protease inhibitors will so suppress viral multiplication that no resistance mutations can develop. However, this argument fails to account for those viruses that were already resistant at the beginning of treatment. Even in the absence of any strains with resistance mutations at baseline, polymorphisms of the virus that existed before treatment may permit selection of strains with increased enzymatic efficiency, making higher drug levels necessary for the same virucidal effect. In addition, it is not known whether tissue levels of drugs are sufficient to prevent the emergence of resistant virus. In these respects, the presence of a syncytium-inducing (SI) phenotype of virus in the lymph node tissues of two untreated patients in the presence of a nonsyncytium-inducing (NSI) phenotype of virus in their plasma is of great concern [8]. If viral resistance occurs, will there be a sufficient number of new agents for new, effective regimens? Understanding of the molecular structure of HIV and the discovery and development of antiviral drugs have occurred at such an astounding pace it is fair to say that no other field of medicine -has advanced so rapidly and to such depth in the past 50 years that it is tempting to assume that this pace will continue. On the other hand, past success does not guarantee future results in any field of human endeavor, and it is thus imprudent to assume that successful antiviral drug development will continue at the present rate. It is also important to note that concerns about whether costs will be recovered might deter pharmaceutical companies from further development of antivirals; Merck & Co. spent almost $1 billion in the development of crixivan. What evidence supports the assumption that long-term decreases in phiv RNA levels will be accompanied by decreases in viral burden in tissues? Only a few studies regarding this question have been published in peer-reviewed journals; all of these studies were short-term and involved such small numbers of patients as to raise questions about the wisdom of generalizing the results. Pantaleo and co-workers [9] were unable to demonstrate a correspondence between levels of HIV-1 in simultaneously obtained samples of plasma and lymph nodes in nine patients with early or intermediate-stage disease, but no samples were obtained at different points in time from the same patient. Faust et al. [10] observed levels of HIV RNA that were up to 10,000 times higher in lymphoid tonsillar biopsy specimens than in plasma [10]. Lafeuillade et al. [11] studied the relation between plasma levels and lymph node levels of HIV in four patients treated with three drugs [11]. The rate of virus clearance was four times higher in plasma than in lymph nodes. These authors suggested that the difference might be due to either uneven distribution of drug in tissues or the presence of long-lived infected cells in lymph nodes. Cohen et al. [12] studied six patients in whom monotherapy with zidovudine was changed to combination therapy by the addition of didanosine. After 8 weeks of therapy, the mean number of HIV DNA copies did not decrease significantly in either peripheral blood mononuclear cells (PBMCs) or lymph nodes; one of these patients had a decrease of 10-fold, and another had a decrease of approximately eightfold in HIV DNA in PBMCs, yet neither patient had a change in the number of HIV DNA copies in lymph node cells. Cohen et al. [12] measured HIV replication in the PBMCs and lymph node mononuclear cells by extracting the RNA and then assaying the HIV RNA. There were substantial, concor-

3 118 Fessel CID 1997;24 (February) dant decreases in HIV replication rates in both blood and lymph node cells in four of the six patients; however, in the other two patients, the levels of HIV RNA in lymph nodes either remained unchanged despite a rise in the blood cell levels or rose despite unchanged blood cell levels. In another study, Cohen et al. [13] compared viral replication rates in lymph node cells with those in plasma in eight patients who started receiving zidovudine. After 8 weeks of therapy, there was a 3.7-fold (nonsignificant) reduction in viremia, yet the level of HIV RNA in lymph node cells was unchanged; two patients had dramatic decrements in plasma viremia without corresponding decreases in lymph-node-cell levels. In summary, the results from these few studies that have been published in peer-reviewed journals and that were conducted with only 18 patients treated for a few months hardly substantiate the assertion that long-term suppression of phiv RNA will result in suppression of tissue levels of the virus. Thirty-three additional patients in four separate studies were described at the Third Conference on Retroviruses and Opportunistic Infections [14]; the results of these studies were similar to those of the published studies in that there were both concordant and discordant changes when the levels of HIV RNA in plasma were compared with those in lymph nodes. I will note the discordant findings herein, since these are the ones that fail to support the assumption that long-term decreases in phiv RNA levels will be accompanied by decreased tissue levels. In one study, the HIV RNA levels in tonsillar tissue from 10 patients did not correspond with those in PBMCs or plasma and fluctuated widely between a 53-fold decrease and a 225- fold increase from measurement to measurement in the same individual [15]. In another study of eight patients, one patient had no reduction in HIV RNA in lymph node cells despite a sevenfold decrease in plasma levels after 6 months of therapy, and another patient had only a fourfold reduction in the level of HIV RNA in lymph node cells despite a 34-fold reduction in plasma levels [16]. A disturbing finding was that two of 11 patients had phenotypic discordance; virus cultured from lymph nodes was SI, whereas that from plasma and PBMCs was NSI [8]. It may also be noted that no correlations have been made between phiv RNA levels and the viral burden in sequestered tissues such as those of the CNS. Of even more concern is the observation that virus in both the blood and the CSF of one patient was susceptible in vitro to zidovudine before therapy was initiated; however, a resistance mutation at the 215 codon appeared in the CSF isolate but not in the blood isolate after 24 months of zidovudine therapy [17].This finding reinforces the concern that despite adequate suppression of phiv RNA, inadequate levels of virucidal drug in tissues might encourage the emergence of resistant virus. Since this review was written, a second example of this has been published (Sei S, et al. J Infect Dis 1996; 174:1200-6). Another observation that shows discordance between phiv RNA levels and the CD4+ lymphocyte count is the pattern of response after initiation of therapy with three drugs that include a protease inhibitor. For most patients, the initial response to combination therapy that includes a protease inhibitor is a decrease in the viral RNA level, with a rise in the CD4 + lymphocyte count; however, the viral RNA level later rises somewhat, yet the blood CD4 + lymphocyte count does not fall and may even rise. This phenomenon must be regarded as a major paradox if the levels of virus in plasma and of CD4+ lymphocytes in blood closely reflect the levels in lymph nodes. In the studies of therapy with high-dose ritonavir [18], the decrement in HIV RNA among the patients was at a nadir of 1.2 log at 4 weeks. The levels then started to rise, but at 20 weeks when the HIV RNA levels had increased by 50% over the initial decrement, the CD4 + lymphocyte counts started to rise a second time by another 150/mm 3; this rise peaked at 24 weeks, and the counts remained the same until the end of the study at 32 weeks. A similar observation was made by Erice et al. [19] among patients treated with high doses (4 mg/[kg d]) of stavudine for 4 weeks [19]. After the initial 4 weeks, the phiv RNA levels decreased by 0.8 log/ml, and the CD4+ lymphocyte counts increased by 68/mm 3. However, 4 weeks later, during which time no patient was retreated, phiv RNA levels returned to baseline, and CD4 + lymphocyte counts continued to rise by a further 26/mm 3 (i.e., 38% above the initial increment); by week 12, most patients had received a second course of stavudine therapy, and although phiv RNA remained at its baseline level, the CD4 + lymphocyte counts remained at their initially increased level. A possible explanation of this apparently paradoxical response is that the virus has become noninfectious, but this phenomenon has never been demonstrated. The relevance of the phiv RNA level depends on both the concomitant virus levels in tissue and the plasma antibody level. The significance of virus circulating as a complex with antibody obviously differs from that of free virus. Hogervorst et al. [20] studied 14 patients who had phiv RNA levels of > 10,000 copies/ml 1 year after seroconversion [20]. Infection in the five patients who had high (> 1:10,000) titers of antibody progressed slowly, whereas that in six of the nine with lower titers of antibody progressed rapidly. If these figures are generally applicable, then 35% of patients with HIV RNA levels of > 10,000 copies/ml also have high antibody levels and infections that progress slowly. It is noteworthy that none of the eight patients with HIV RNA levels of ,000 copies/ml had high (> 1:10,000) antibody levels, and six of these eight patients had infections that progressed rapidly. How dangerous are high levels of phiv RNA in the presence of acceptable CD4+ lymphocyte counts? Although the baseline phiv RNA level is more accurately prognostic than is the baseline CD4 + lymphocyte count [7] and changes in phiv RNA levels both presage progression of infection in some patients and are possibly more accurate indicators than are changes in CD4+ lymphocyte levels, a legitimate question is whether it is mandatory to treat all patients on the basis of the

4 CID 1997;24 (February) HIV RNA in Plasma 119 viral load changes rather than to await decreases in CD4+ lymphocyte counts. In the widely quoted paper by Mellors et al. [7] there were no cases of AIDS in patients with baseline CD4 + lymphocyte counts above 322/mm 3 until almost 15 months after entry into the study, although no patient had received treatment for the first 6 months of the study, and only 41% received treatment after 6 months [7 (fig 2c)]. Today, most patients with CD4+ lymphocyte counts between 322/mm3 and 500/mm3 receive treatment with at least one drug, and usually two drugs; it is likely that the AIDS-free interval would now be substantially longer than 15 months for these patients and might possibly be double that time. Even 15 months is an adequate amount of time to make a decision to change treatment if a decrease in the number of CD4+ lymphocytes occurs during this period. It has been argued that there is an advantage to treating patients with rapidly progressing disease 15 months sooner than is possible on the basis of a CD4+ cell decline, but this argument does not account for the fact that only a tiny fraction of patients (-5% of those described by Mellors et al.) had AIDS at 15 months after entry into the study (this is consistent with the estimate, vide infra, that 3.5% of HIV-infected patients per year develop AIDS); therefore, the remainder of patients would be treated in advance for a much longer time, during which drug toxicity and viral resistance might occur. In their update on the dynamics of HIV infection, Havlir and Richman [6] discussed the issue of intervening with treatment earlier than is presently recommended; they offered the opinion that current guidelines, which suggest withholding antiretroviral therapy from all patients with CD4+ lymphocyte counts >500/mm3, should be revised. These authors strongly emphasized this opinion as follows: "Some experts advocate withholding therapy in patients with fewer than 10,000 HIV RNA copies/ml of plasma. The data to justify this recommendation are not available, and this approach should be viewed with caution" [6]. The problem is that the data justifying widespread administration of antiretroviral therapy to patients with CD4+ lymphocyte counts of >500/mm 3 are also not available, whereas there are many data that justify withholding therapy from most patients with these higher CD4 + lymphocyte counts, except in the setting of a research protocol. Havlir and Richman [6] stated that the infections in most patients who continue to have HIV RNA levels of 1, ,000 copies/ml progress to AIDS in 8-10 years; this presumably is the premise behind their recommendation that these patients be aggressively treated. The statement appears to derive from the study of Mellors et al. [7], but it should be noted that the patients to which it refers had all remained untreated for 6 months, and only 41% received antiviral therapy at any time. In fact, only % of HIV infections per year progress to AIDS and the rate is closer to the lower of these two percentages for patients with less advanced disease. For example, I studied the outcomes for 96 patients after 2 years who initially had CD4 + lymphocyte counts of >630/mm 3 : six had developed an AIDS-defining condition, for an annual rate of 3.1% [21]. Phillips et al. [22] observed for up to 8 years 112 HIV-infected hemophilic patients for whom the approximate dates of seroconversion were known; by using the observed rate of progression to AIDS, they estimated that 73% of these patients would develop AIDS at 15 years, for an average rate of 4.99% of patients per year. During the first 5.4 years, zidovudine was not available; 22 patients developed AIDS, for a rate of 3.6% of patients per year. Darby et al. [23] also observed 858 hemophiliacs (age range, years) for whom the approximate dates of seroconversion were known and found that the annual rate of the development of AIDS over 10 years was 4.1% of patients; the annual rate of observed deaths minus expected deaths in this population of patients was 1.2%. Thus, the rate of progression to AIDS is at worst 5% per year among HIV-infected patients, and a rate of 3.5% per year might be a fair overall estimate for patients with early HIV infection; 8 years after infection, the percentage of patients with AIDS would be between 28% and 40%. This point is not a mere academic quibble. If most patients develop AIDS 8-10 years after the primary infection as stated by Havlir and Richman [6], then early aggressive treatment of all patients might be justified. However, if the percentage of patients who develop AIDS at 8 years after primary infection is only 28%, then it is difficult to accept the recommendation of these authors that patients with high levels of HIV RNA be treated despite their CD4+ lymphocyte counts. (Havlir and Richman did not specify what they meant by a high HIV RNA level; however, their discussion casts doubt on a level as high as 10,000 copies/ml, although this was the cutoff for the baseline level that separated those patients for whom the prognosis was bad from those for whom it was better in the study reported by Mellors et al. [7].) Approximately 50% of patients with CD4+ lymphocyte counts of >500/mm 3 have HIV RNA levels that are >10,000 copies/ml [7]; this degree of viral load is a leading indicator of rapid progression to AIDS but is observed about 2 years in advance of the onset of AIDS in < 10% of patients [7]. If the fact that viral load does not predict all rapid progressions [6] is ignored and it is assumed that therapy will prevent all instances of rapidly progressing HIV disease, treatment given for 1 year to the 50% of patients with CD4+ lymphocyte counts above 500/mm 3 and HIV RNA levels above 10,000 copies/ml would benefit only 7% of them. It would take 14 years before 100% of the treated patients would benefit. From the national perspective, these calculations can be applied to the estimated 1 million HIV-infected persons in the United States, of whom 20% (200,000) have CD4+ lymphocyte counts of >500/mm 3. If treatment with three drugs that include a protease inhibitor were given to the 50% (100,000) who also have HIV-RNA levels of > 10,000 copies/ml, the cost of the drugs alone at $10,000 $15,000 per patient per year [24]

5 120 Fessel CID 1997;24 (February) would be, using the lower figure, $1 billion per year. This figure is exclusive of the costs of the viral load assays, which should be done at least four times a year (in practice they would be done more often because they should be repeated if a substantial rise in viral load suggests the need to change treatment) at a minimum cost of $90 per assay; the same frequency would apply to visits to physicians, at a cost of $50.00 per visit. These amounts add $56 million annually to the national bill, which, with the addition of the drug costs, would become $1.056 billion per year, or $150,857 per year for each rapid progression of HIV disease that was averted during that year. These are minimum estimates and could be 50% higher. Unaccounted factors include the toxicities to the 93% of patients in the treated cohort whose disease does not progress, the financial costs of monitoring for and treating these toxicities, and the fact that it has not been demonstrated in a clinical trial that treatment will in fact avert all rapid progressions of HIV disease. It is legitimate to ask whether careful monitoring of CD4 + lymphocyte counts and prompt intervention with two or three drugs when a significant decline in the CD4 + lymphocyte count occurs might not benefit those with rapidly progressing disease as much as earlier intervention that is based on viral load. The issue is not merely fiscal (although the expenditures are impressive) but is more one of whether it is proper to give powerful drugs to so many patients whose disease has not yet progressed; toxicities as well as viral resistance may occur. Evidence that careful monitoring of CD4 + lymphocytes remains a legitimate approach in the management of HIV infection may be seen in the paper by Mellors et al. [7 (fig 2c)], in which no cases of AIDS occurred in patients with baseline CD4 + lymphocyte counts of >322/mm 3 until almost 15 months later, even though none of the patients had received treatment during the first 6 months, and only 41% received treatment at any time. This finding suggests that for patients with this level of CD4 + lymphocytes, HIV RNA testing adds little to the decision about adjusting therapy [7]. Mellors et al. also showed that 100% of patients with CD4+ lymphocyte counts of >385/mm 3 had survived longer than 2 years. The Cox proportional hazards analysis showed that the relative hazard of death was 1.33 for a 100-cell decrease from the baseline CD4+ lymphocyte count and 1.57 for each threefold increase in the concentration of HIV RNA from baseline. It is arbitrary to compare a threefold change in the HIV RNA level with a 100-cell change in the CD4+ lymphocyte count. What the studies clearly demonstrate is that a decrease in the number of CD4 + lymphocytes is as useful for assessing the need to change therapy as is a rise in the HIV RNA level and that for patients with CD4 + lymphocyte counts of >322/mm 3 at baseline, there is a substantial window of time during which treatment may be directed toward raising the CD4 + lymphocyte count without risking the development of AIDS or compromising survival. This finding was confirmed in a smaller study of 73 Swiss patients [25]. Survival was similar for the first 6-9 months in patient groups defined by HIV RNA tertiles. Investigators who propose that HIV RNA be made the target for therapy in HIV-infected patients cite three major reports in justification: the virological studies that were done in the Veterans Administration trial [26]; the virology substudy of ACTG (AIDS Clinical Trials Group) 175 [27]; and the report of increased survival among patients in studies of ritonavir [28]. Careful examination of the Veterans Administration study shows that a 10% rise in CD4 + lymphocyte counts was associated with a better clinical outcome than was seen with a 75% decrease in HIV RNA levels. The clinical outcomes for patients with both increases in CD4 + lymphocyte counts and decreases in HIV RNA levels were the same as those for patients who had increases in CD4 + lymphocyte counts alone. Despite these findings, the authors concluded that the objective of treatment is to reduce HIV RNA levels as much as possible [26]. However, their data better support the conclusion that the objective of therapy is to raise the CD4 + lymphocyte level as much as possible; the fact that almost every manifestation of AIDS depends on a decrease in the number or function of CD4 + lymphocytes also supports this conclusion. The virological findings from ACTG 175, which were reported by Katzenstein et al. [27], were modestly interpreted by these authors as showing that the HIV RNA level may provide a basis for more targeted, rational use of antiretroviral therapies in early HIV infection and that the CD4 + lymphocyte count provides useful but limited information about risk for clinical progression of disease. Other investigators have used the findings of ACTG 175 to support the more aggressive stance that HIV RNA is the preferred target of therapy [6, 29, 30], but careful scrutiny of the findings from the ACTG 175 virological substudy questions the strength of this conclusion. First, the finding of ACTG 175 that baseline CD4 + lymphocyte counts were not associated with the hazard of disease progression after adjustment for HIV RNA levels and NSI phenotype contradicts many prior findings that baseline CD4+ lymphocyte counts are strongly correlated with progression [31]. Analysis of the results of ACTG 116A showed that the baseline CD4 + cell count and HIV RNA level each remained independently predictive of disease progression after simultaneous adjustment for other putative predictors [32]. As mentioned above, the Cox proportional hazards analysis of the MACS (Multicenter AIDS Cohort Study) cohort showed that the relative hazard of death associated with a 100-cell decrease from baseline in the CD4 + lymphocyte count remained statistically significant even after controlling for HIV RNA levels [7]. Similarly, in the smaller Swiss study, where mean follow-up was 14.6 months, inclusion of HIV RNA levels, CD4 + cell counts, and clinical stages as covariates in the proportional hazards model revealed that CD4 + cell counts were a strong, statistically significant predictor of death, while the predictive value of the HIV RNA level was no longer clinically significant [25]. For the 620 patients in the North

6 CID 1997; 24 (February) HIV RNA in Plasma 121 American delavirdine study, there was a correlation between phiv RNA levels and CD4 + cell levels at baseline, and in the multiple regression model each marker provided additional information independent of that given by the other [33]. Next, the authors of the ACTG 175 substudy report themselves pointed to some paradoxical findings that at the least show that the relation between suppression of HIV RNA levels and progression of HIV disease is imprecise. Whereas the overall results showed a strong association between a decrease in phiv RNA levels and a reduction in the risk of progression, the clinical endpoints with didanosine alone and didanosine plus zidovudine were comparable, but the decrease in the phiv RNA level was substantially greater with the combination treatment. The clinical endpoints were better with zidovudine plus didanosine than with zidovudine plus zalcitabine; however, suppression of phiv RNA levels was the same with both regimens. Finally, statistical manipulation of the data showed a 90% reduction in the hazard of progression to AIDS or death after week 56 when there was a 1-log suppression of HIV RNA levels between baseline and week 56; however, the same analysis yielded the biologically implausible conclusion, inconsistent with the findings from the Veterans Administration study, that a rising CD4 + lymphocyte count at week 56 was not associated with a decreased hazard of progression to AIDS. Not only did the analysis of the data from ACTG 175 show that the prognostic value of CD4 + lymphocyte counts at baseline was negated when "corrected" for HIV RNA levels, but it also showed that immunologic improvement, as reflected by a rising CD4+ lymphocyte count between baseline and week 56, did not reduce the hazard of progression. It is noteworthy that in the delavirdine study, the CD4 + lymphocyte count and phiv RNA level measured at baseline before the trial treatment was begun were substantially less predictive of the clinical outcome than were the most recent values [33]. Thus, the studies most frequently cited by those who view HIV RNA as the preferred target of therapy contain mutual inconsistencies that weaken the case for phiv RNA being a better monitoring tool than are CD4 + lymphocytes. A third piece of evidence cited by those who propose HIV RNA as the preferred target of therapy is the report of increased survival among patients given ritonavir [28]. Levy's comment is noteworthy: "Can one. really report a 50% increase in survival based on only 6 months of treatment and results that reflect 4.8% (treated) vs 8.4% (untreated) of the subjects studied?" [4]. Moreover, in the ACTG protocols 116B/117, for which patients with advanced disease (median CD4 + cell count, 85/mm 3) were studied, progression of disease occurred in up to 60% of patients with fewer than 10,000 HIV RNA copies/ml [34]. A final comment concerns use of the HIV RNA assay for patients with advanced disease. Such patients tend to have the highest levels of phiv RNA, but these patients are the very ones who are now generally given three-drug therapy, which usually includes a protease inhibitor; we have long known that the prognosis is worst for patients with CD4 + lymphocyte counts of <200/mm3. What is the benefit of testing for HIV RNA levels if they merely confirm this knowledge and we would initiate the most aggressive therapy with or without this additional marker anyway? When monitoring the effects of therapy in such patients, it seems superfluous to obtain repeated measurements of phiv RNA, provided there has been a satisfactory rise in CD4 + lymphocyte levels. It is when there is an unsatisfactory response of CD4 + lymphocytes in these patients, as well as in those with less advanced disease, that measurement of phiv RNA is useful and justified. In summary, the following statements regarding the clinical value of the phiv RNA test can be supported by the existing data: (1) The virology of HIV disease involves tissue sites as well as blood. (2) The relation between levels of virus found in the blood and the levels found in the tissues has not been properly elucidated; current evidence from studies of only a few patients suggests a low correlation between levels of virus in blood and those in tissues. (3) A single high level of phiv RNA found at any time beyond 1 year after primary infection suggests that the patient might have rapidly progressing disease; however, there is a window of at least 15 months before AIDS will develop in those patients with CD4 + lymphocyte counts of >321/mm3, despite the phiv RNA level [7]. (4) The prognosis is better for those patients whose HIV RNA levels fall within a few months after treatment has been started than for those whose levels do not fall; the same applies to those whose CD4 + lymphocyte counts rise. (5) No study has yet shown that an approach to therapy that seeks to minimize HIV RNA levels and maintain the low levels is superior to the traditional approach that seeks to maximize CD4 + lymphocyte levels and maintain these; the reason for this is probably that when HIV RNA levels fall in response to therapy, CD4 + cell levels usually rise a couple of months thereafter. Moreover, no study that was designed specifically to compare these two approaches has yet been completed. It seems reasonable to draw the following conclusions. First, further evidence is required to demonstrate convincingly that the phiv RNA level is either the best available ongoing target for therapy or a better target than the CD4 + lymphocyte count. Second, the precise role of phiv RNA testing in the clinical management of HIV infection remains to be defined. Until this role has been defined, the test is best performed on a case-bycase basis as a guide to management rather than as an absolute target for therapeutic manipulation. Third, with respect to the proposal that therapy be initiated when CD4 + lymphocyte counts exceed 500/mm 3 if plasma HIV RNA levels are above 10,000 copies/ml, the risks of side effects and the financial costs to the patients whose disease will not progress rapidly to AIDS i.e., the majority are too high. References 1. De Gruttola V, Wulfsohn M, Fischl MA, Tsiatis A. Modeling the relationship between survival and CD4 lymphocytes in patients with AIDS and

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