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Background We assessed long-term virological and immunological effectiveness and tolerability of CAB+RPV as a switch strategy in clinical practice.Material and Methods This observational study enrolled virologically suppressed people with HIV (PWH) switching to CAB+RPV within the ODOACRE multicenter cohort. Clinical and virological data were collected. Kaplan–Meier analysis evaluated time to virological failure (VF) and treatment discontinuation (TD). VF was primarily defined according to a Delphi-like criterion as two consecutive HIV-RNA ≥200 copies/mL or a single ≥1000 copies/mL (CVF-200). A more stringent sensitivity definition was also applied (two consecutive HIV-RNA ≥50 copies/mL or a single ≥1000 copies/mL; CVF-50). Predictors of VF were assessed using Cox regression based on CVF-200 with parallel sensitivity analyses using CVF-50.Results Overall, 490 PWH were included: 398 (81.2%) were males, with a median age of 52 years (IQR 43-59). Full population characteristics are shown in table 1.During 809.8 PYFU (median follow-up of 1.85 years), 5 CVF-200 occurred (0.62/100 PYFU); using the stringent definition, 15 CVF-50 were observed (1.85/100 PYFU). Freedom from VF was 98.9% and 98.5% at weeks 48 and 144 using the CVF-200 definition and 97.5% and 95.1% using the CVF-50 definition.In Cox models, previous VF on NNRTI-based ART was associated with higher risk of VF using CVF-50 (aHR 28.3, 95%CI 2.5-322.8, p=0.007), after adjusting for viral subtype and BMI. With CVF-200, only a non-significant trend was observed for subtype A1/A6 (aHR 13.7, 95%CI 0.8-232.5, p=0.070).Regarding tolerability, during 815.2 PYFU we observed 69 TD (8.46/100 PYFU). Main reasons were injection-site reactions (24, 4.9% of total population), treatment intensification (17, 3.5%), CNS toxicity (7, 1.4%), PWH request (6, 1.2%), other toxicity (3, 0.6%), pregnancy (3, 0.6%), death (3, 0.6%), other/unknown (6, 1.2%). Estimated probability of maintaining CAB+RPV was 89.5% at week 48 and 80.2% at week 144.CD4+ count and CD4/CD8 ratio progressively improved, with significant increases at weeks 96 (both) and 144 (CD4/CD8). CD4+ gain was greater in PWH with lower baseline CD4 (per +10 cell/mm3: B -2.3, p=0.001) and in women (B -97.0, p=0.014) after adjusting for age, CD4+ cell nadir and number of previous ARV lines. Women also showed greater improvement in CD4/CD8 ratio (B -0.15, p=0.011). No clinically relevant changes in BMI or SCORE-2 were observed, while HDL showed a modest but significant increase during follow-up.Conclusions In this large real-world cohort, CAB+RPV showed durable effectiveness and good tolerability through 144 weeks. However, the markedly increased risk of VF observed in PWH with prior NNRTI failure suggests that this switch strategy should be used with caution and generally avoided in this population. Continued immunological improvement supports the robustness of CAB+RPV in appropriately selected patients.Abstract OC38 Table 1Population characteristics at baseline