Definition and Levels of Lactic Acidemia
Lactic acidemia (hyperlactatemia) is a well-described potential complication resulting from the use of nucleoside reverse transcriptase inhibitors (NRTIs) and is defined as a venous lactate level greater than 2 mmol/L (18 mg/dL) in conjunction with a normal arterial pH.[,] Experts have defined the severity of lactic acidemia based on the measured venous lactate level and the presence or absence of characteristic clinical symptoms.[,] Low-level lactic acidemia refers to a venous lactate level of 2 to 5 mmol/L (18 to 45 mg/dL), moderate-level lactic acidemia consists of a venous lactate of 5 to 10 mmol/L (45 to 90 mg/dL) without signs or symptoms that suggest lactic acidemia, and severe-level lactic acidemia is defined as a venous lactate greater than 10 mmol/L (90 mg/dL) regardless of symptoms, or a level of 5 to 10 mmol/L (45 to 90 mg/dL) in the presence of signs and symptoms that suggest lactic acidemia (Figure 1). Lactic acidosis, the most severe degree of lactic acidemia, consists of venous lactate greater than 2 mmol/L (18 mg/dL) and an arterial pH less than 7.30. For most HIV-related reports of lactic acidemia, investigators have not reported the arterial pH and thus the subsequent discussion will focus on lactic acidemia.
Risk Factors and Incidence
Investigators have identified multiple risk factors associated with the development of lactic acidemia: use of stavudine (Zerit), didanosine (Videx or Videx EC), or high-dose zidovudine (Retrovir); use of stavudine plus didanosine; use of didanosine plus hydroxyurea (Hydrea); use of didanosine plus ribavirin (Rebetol, Copegus) in patients with hepatitis C virus infection; use of stavudine plus didanosine during pregnancy; female gender; creatinine clearance less than 70 mL/min; obesity; and advanced immunosuppression (Figure 2).[,,,,,,]. Among the NRTIs, stavudine appears to cause lactic acidemia more frequently than didanosine or zidovudine and most of the cases of zidovudine-related lactic acidemia involved high-dose zidovudine used as monotherapy.. Lactic acidemia directly attributable to tenofovir (Viread), abacavir (Ziagen), lamivudine (Epivir), or emtricitabine (Emtriva) appears to occur very rarely, if at all. Several studies, when taken together, have shown detection of asymptomatic or mildly symptomatic lactic acidemia in 8 to 21% of patients receiving at least 1 NRTI.[,,,,] Lactic acidemia with significant symptoms occurs less frequently, with an estimated incidence of 1.5 to 2.5% among persons takingNRTIs.These estimates, however, are primarily based on data collected during an era of antiretroviral therapy that involved much greater use of stavudine and didanosine than currently used in clinical practice. Thus, contemporary rates of lactic acidemia are presumably lower than previously reported.
Human mitochondria are sausage-shaped organelles that play a vital role as the energy powerhouse of the cell. To understand how lactic acidemia develops, it is important to first understand the complex process of how mitochondria function (Figure 3).[,,]. All human cells, except for erythrocytes, have numerous intracytoplasmic mitochondria, and each mitochondrion contains multiple copies of DNA. Within the mitochondria, DNA polymerase gamma serves as the sole DNA polymerase responsible for mitochondrial DNA (mtDNA) replication. The mitochondria synthesize multiple components of the oxidative phosphorylation system. The oxidative phosphorylation system resides on the inner mitochondrial membrane and consists of five complexes, with each complex comprised of multiple subunits. In four of the five oxidative phosphorylation system complexes, mtDNA encodes for at least one of the subunits. The oxidative phosphorylation system couples the transfer of electrons (in the electron transport system) with the movement of H+ to the intermembrane space, creating an energy rich proton gradient. The movement of H+ back into the mitochondria releases free energy used to convert ADP to ATP. The ATP is then transported out to cytosol, where it is converted to ADP, thereby generating energy for the cell. The process of oxidative phosphorylation utilizes the conversion of NADH to NAD+ and FADH2 to FADH.
The generation of energy by the mitochondrial depends on glucose and triglycerides serving as critical energy substrates for the mitochondria. The process of glycolysis in the cytosol generates pyruvate, which then can enter the mitochondria, or can react with NADH to form lactate in the cytosol. The fate of the pyruvate depends on conditions within the cell, with aerobic conditions favoring the entry of pyruvate into the mitochondria for further metabolism, and anaerobic conditions (or disruption of the oxidative phosphorylation system) favoring the conversion of pyruvate to lactate. The pyruvate that enters the mitochondria is converted to acetyl Co-A, which then feeds into the Krebs's cycle. The acetyl Co-A eventually is metabolized to CO2, H2O, and ATP and this process also reduces NAD+ to NADH and FADH to FADH2. The NADH and FADH2 are then oxidized during oxidative phosphorylation. Overall, a dynamic equilibrium exists in the cytosol between pyruvate and lactate, and the enzyme lactate dehydrogenase (LDH) catalyzes the conversion in both directions. Moreover, the specific direction of conversion depends on the amount of NAD+ produced from NADH during oxidative phosphorylation: with normal oxidative phosphorylation, adequate NAD+ is produced and lactate flows to pyruvate. Any process that disrupts oxidative phosphorylation will diminish the production of NAD+, increase the ratio of NADH to NAD+, and thus favor the conversion of pyruvate to lactate.
Mitochondria Dysfunction with NRTIs
The proposed mechanism for the development of antiretroviral-related hyperlactatemia has centered on medication-induced inhibition of mitochondrial DNA polymerase gamma, leading to depletion of mtDNA and diminished capacity of the oxidative phosphorylation system. Data from multiple studies have shown that antiretroviral therapy-näive, HIV-infected persons have diminished mtDNA levels when compared with age-matched HIV-negative controls. Moreover, one study found that treatment-näive HIV-infected individuals had a 34% decrease in the mitochondrial:nuclear DNA ratio in peripheral blood cells when compared with HIV-negative controls (Figure 4). Those HIV-infected patients who received stavudine-containing antiretroviral therapy and developed symptomatic lactic acidemia had a further reduction in mitochondrial:nuclear DNA ratios, a reduction that was reversed when antiretroviral therapy was discontinued (Figure 5). In contrast, other studies have shown some antiretroviral regimens actually improve mtDNA levels. Thus, it appears that antiretroviral therapy can have either a negative or positive effect on mitochondrial function, depending on the specific NRTIs used in the regimen. Investigators recently examined the impact of specific NRTIs on the level of mtDNA synthesis in three cell types (skeletal muscle, hepatic, and renal cells) and reported a hierarchy of effect, with zalcitabine (Hivid) causing the greatest decrease in mtDNA synthesis, followed by didanosine, then stavudine, then zidovudine. The least effect was observed with lamivudine, abacavir, and tenofovir. Earlier work also established that zalcitabine, didanosine, and stavudine have the greatest negative impact on DNA polymerase gamma.
Among the human DNA polymerases, only DNA polymerases beta and gamma are inhibited by the NRTIs. Because the DNA polymerases alpha and delta are the polymerases responsible for nuclear DNA replication, the NRTIs do not affect normal cell cycle replication. Because DNA polymerase gamma is the only DNA polymerase in the mitochondria, inhibition of this enzyme can diminish mtDNA synthesis and thereby blunt the generation of mtDNA that encodes critical subunits of the oxidative phosphorylation system complexes (Figure 5). The potential consequence is a dysfunctional oxidative phosphorylation system, a decreased conversion of NADH to NAD+, an increased ratio of NADH to NAD+, and a shift in cytosolic conversion of pyruvate to lactate. The liver serves as the primary organ that clears lactate from the circulation by converting lactate to pyruvate in hepatocytes, thus impaired liver function will compound the problem of increased lactate production. Indeed, the development of chronic hyperlactatemia generally occurs only in the setting of impaired mitochondrial function in the hepatocytes.
Because some reports have described NRTI-related hyperlactatemia without changes in mitochondrial DNA levels, investigators have explored alternative mechanisms for the development of mitochondrial dysfunction. One group has reported NRTI-induced decreases in transcription of mitochondria mRNA in adipose tissue in the absence of depleted mitochondrial DNA levels. They proposed that through a process of mitochondrial fine tuning, the mitochondria respond to intramitochondrial accumulation of phosphorylated NRTIs by down-regulating mtRNA transcription; the depletion in the levels of mtRNA has the same effect that decreasing mtDNA would have, namely impairing the mitochondrial oxidative phosphorylation capacity.
Clinical Symptoms of Lactic Acidemia
The onset of lactic acidemia typically begins months, or even years after starting an antiretroviral regimen that includes one or more NRTIs. One series noted the onset of symptoms a median of 4 months after starting antiretroviral therapy. The most common symptoms include nausea, vomiting, abdominal pain, weight loss, fatigue, myalgias, and features of hepatic dysfunction, including tender hepatomegaly, peripheral edema, ascites, and encephalopathy.[,,,,] Later symptoms may include dyspnea and cardiac dysrhythmias. Some patients also have signs or symptoms of other mitochondrial toxicities, such as peripheral neuropathy and lipoatrophy. Unfortunately, many of the symptoms associated with hyperlactatemia are non-specific and one study found that mild to moderate symptoms consistent with hyperlactatemia did not accurately correlate with serum lactate levels.
Routine monitoring of serum lactate levels in an asymptomatic patient is not recommended. Establishing the diagnosis of lactic acidemia requires the documentation of a serum lactate level greater than 2 mmol/L (18 mg/dL). Obtaining an accurate measurement of the serum lactate level requires specific counseling for the patient prior to the blood draw, as well as using appropriate technique during the blood draw and properly handling the specimen. Specifically, the patient should remain well hydrated and avoid vigorous exercise for at least 24 hours prior to blood draw. Prior to and during the blood draw, the patient should avoid fist clenching and should not have a tourniquet applied for a prolonged period. The sample should be placed in a pre-chilled fluoride-oxalate tube (gray-top), transported to the laboratory on ice, and processed within 4 hours of phlebotomy. If the patient has an elevated serum lactate level, the clinician should consider other causes of increased lactate, including inappropriate sample collection or processing, dehydration, vigorous exercise, alcohol intoxication, sepsis, renal failure, pancreatitis, hyperthyroidism, and toxicity from medications other than antiretrovirals.
Measurement of serum bicarbonate, anion gap, and hepatic transaminase levels should also be performed if the clinician suspects a diagnosis of lactic acidemia. Although many patients with lactic acidemia have a decreased serum bicarbonate level and an increased anion gap, the presence of a normal bicarbonate or normal anion gap does not rule out lactic acidemia. Several research studies have reported the measurement of mitochondrial DNA levels, but these tests are not recommended for routine evaluation in clinical practice. Hepatic imaging studies often show evidence of hepatic steatosis.
Management of lactic acidemia depends on the measured serum lactate level and the presence or absence of clinical symptoms (Figure 6). Patients who have a serum lactate level greater than 10 mmol/L (90 mg/dL), regardless of clinical symptoms, and those with a lactate level between 5 to 10 mmol/L (45 to 90 mg/dL) in conjunction with clinical symptoms associated with lactic acidemia should discontinue antiretroviral therapy.[,] Although it is most important to discontinue the NRTIs, the patient should stop all other antiretroviral medications to avoid development of resistance. Firm recommendations for asymptomatic patients who have a lactate level between 5 to 10 mmol/L (45 to 90 mg/dL) do not exist, but many experts would recommend discontinuing antiretroviral therapy, or to switch the patient's NRTI to an agent not associated with the development of lactic acidemia, followed by frequent monitoring of the lactate level. Patients with a serum lactate level between 2 to 5 mmol/L (18 to 45 mg/dL) should have frequent monitoring if they have symptoms consistent with lactic academia and those with pronounced symptoms should discontinue antiretroviral therapy. For asymptomatic patients with a serum lactate level between 2 and 5 mmol/L (45 mg/dL), it is reasonable to continue therapy with monitoring of serum lactate levels.
Multiple agents have been tried for therapy of non-HIV-infected persons with inherited mitochondrial diseases, but no systematic trials have been performed in HIV-infected patients with lactic acidemia. Several case reports have suggested clinical benefit in HIV-infected patients with lactic acidemia after administering riboflavin (vitamin B2), ubiquinone coenzyme Q10), and thiamine. Overall, these agents remain unproven in this setting, though they are safe and inexpensive. The prognosis for patients with lactic acidemia varies depending on the severity of the lactic acidemia; available data suggest a mortality rate greater than 40% in patients with lactate levels greater than 10 mmol/L (90 mg/dl), but a very low risk of death with lactate levels less than 10 mmol/L (90 mg/dl).
Reinitiating Antiretroviral Therapy
After discontinuing the NRTIs, the resolution of lactic acidemia can take months. In one report, patients with lactic acidemia (mean 4.2 mmol/L [37.8 mg/dL]) had a decrease in lactate to less than 2 mmol/L (18 mg/dL) at 3 months after stopping the NRTIs. In general, it is most reasonable delay reinitiating antiretroviral therapy (using a different regimen) until the lactate level has decreased to less than 2 mmol/L (18 mg/dL). Several reports have described patients who successfully reinitiated antiretroviral therapy at a later point, using a different regimen that did not include stavudine or didanosine.[,] Patients who reinitiate antiretroviral therapy should have lactate levels monitored every month for at least 3 months. Preferred NRTIs for patients who have experienced lactic acidemia are tenofovir, abacavir, lamivudine, and emtricitabine. With the array of antiretroviral agents now available, most patients could also receive a NRTI-sparing regimen.