Potential toxicities of HIV therapeutics in the developing infant

Neurotoxic effects of AZT on developing and adult neurogenesis

Azidothymidine (AZT) is a synthetic, chain-terminating nucleoside analog used to treat HIV-1 infection. While AZT is not actively transported across the blood brain barrier, it does accumulate at high levels in cerebrospinal fluid, and subsequently diffuses into the overlying parenchyma. Due to the close anatomical proximity of the neurogenic niches to the ventricular system, we hypothesize that diffusion from CSF exposes neural stem/progenitor cells and their progeny to biologically relevant levels of AZT sufficient to perturb normal cell functions. We employed in vitro and in vivo models of mouse neurogenesis in order to assess the effects of AZT on developing and adult neurogenesis. Using in vitro assays we show that AZT reduces the population expansion potential of neural stem/progenitor cells by inducing senescence. Additionally, in a model of in vitro neurogenesis AZT severely attenuates neuroblast production. These effects are mirrored in vivo by clinically-relevant animal models. We show that in utero AZT exposure perturbs both population expansion and neurogenesis among neural stem/progenitor cells. Additionally, a short-term AZT regimen in adult mice suppresses subependymal zone neurogenesis. These data reveal novel negative effects of AZT on neural stem cell biology. Given that the sequelae of HIV infection often include neurologic deficits—subsumed under AIDS Dementia Complex (Brew, 1999)—it is important to determine to what extent AZT negatively affects neurological function in ways that contribute to, or exacerbate, ADC in order to avoid attributing iatrogenic drug effects to the underlying disease process, and thereby skewing the risk/benefit analysis of AZT therapy.

Prolonged perinatal AZT administration and early maternal separation: effects on social and emotional behaviour of periadolescent mice

Zidovudine (AZT) is an effective treatment in preventing perinatal transmission of HIV-1; however, a continuous re-evaluation of the risk-benefit ratio of human exposure to this drug is suggested by both clinical and animal studies. The objective of this study was to assess the medium and long-term effects of pre-postnatal AZT treatment on mouse social and emotional behaviour and the possible interactions between AZT exposure and disruptions in the mother-infant relationship. Pregnant CD-1 mice were administered per os with AZT (160 mg/kg) from pregnancy day 10, throughout delivery, to lactation day 10. In half of the litters, the offspring was separated from the mother for 3 h from postnatal days 2 (PND2) to PND14. On PND35, a 30-min social interaction test was performed and corticosterone levels were measured at the end of the session. On PND80, long-term effects of AZT on emotionality were assess by means of an elevated plus-maze. Results indicate that, on PND35, previous AZT exposure affected social behaviour of the experimental subjects, reducing aggressive interactions in males, while decreasing investigative behaviours in females. At adulthood, AZT inhibited exploratory behaviour in the plus-maze while increasing the frequency of risk-assessment postures in male mice. As for maternal deprivation, this early manipulation exerted a pro-aggressive effect in adolescent male mice, deprived subjects being overall characterised by higher activity levels and a deficit in habituation, an effect also observed in the plus-maze. A significant interaction between AZT and maternal deprivation was found for affiliative behaviours. As for corticosterone levels, no AZT effect was found, while maternal deprivation tended to reduce elevations of this hormone in response to stressful stimuli. Overall results from this study indicate that both AZT exposure and maternal deprivation induced gender-dependent changes in social and emotional behaviour both during adolescence and at adulthood.

Animal models of anti-HIV drugs exposure during pregnancy: effects on neurobehavioral development

In the last 10 years, zidovudine (AZT) has become the main prophylactic therapy against vertical HIV-1 transmission. AIDS Clinical Trials Group (ACTG) 076 have demonstrated that the administration of AZT to HIV-infected women during their third trimester of pregnancy, trough labor and given orally to babies for 6 weeks, reduced by two-thirds the rate of vertical infection. Although the rapid diffusion of this regimen into clinical practice together with the implementation of HIV counseling and testing practices have dramatically reduced the vertical transmission rate in the US and Western Europe, there is a growing concern on the adverse effects of antiretroviral therapy on the fetus and the newborn. In fact, even though shorter regimen therapies that are less complex and expensive to implement in poor countries have been demonstrated as effective as ACTG 076 regimen, the distribution of the risk of vertical transmission in the developing countries is still very high. Consequently, a large number of unborns will be a candidate to developmental exposure to antiretroviral agents. To date, data on the transplacental mutagenicity, carcinogenicity and mitochondrial dysfunction induced by developmental exposure to AZT have been reported in several animal models. Furthermore, one study reported severe yet few human cases of cardiomyopathy and neurological disease likely associated with mitochondrial dysfunction in uninfected infants of seropositive mothers perinatally exposed to AZT. For all of these reasons, many investigations have been focusing on the assessment of the potential adverse effects of nucleoside reverse transcriptase (RT) inhibitors (NRTI) administration during development. A survey of the main results derived from clinical and animal studies is reported here, focusing on those neurobehavioral studies that have been looking for specific and/or aspecific changes in the nervous system induced by NRTI exposure in utero.