Understanding CYP3A4 and P-gp mediated drug-drug interactions through PBPK modeling - Case example of pralsetinib.

Pralsetinib, a potent and selective inhibitor of oncogenic RET fusion and RET mutant proteins, is a substrate of the drug metabolizing enzyme CYP3A4 and a substrate of the efflux transporter P-gp based on in vitro data. Therefore, its pharmacokinetics (PKs) may be affected by co-administration of potent CYP3A4 inhibitors and inducers, P-gp inhibitors, and combined CYP3A4 and P-gp inhibitors. With the frequent overlap between CYP3A4 and P-gp substrates/inhibitors, pralsetinib is a challenging and representative example of the need to more quantitatively characterize transporter-enzyme interplay. A physiologically-based PK (PBPK) model for pralsetinib was developed to understand the victim drug-drug interaction (DDI) risk for pralsetinib. The key parameters driving the magnitude of pralsetinib DDIs, the P-gp intrinsic clearance and the fraction metabolized by CYP3A4, were determined from PBPK simulations that best captured observed DDIs from three clinical studies. Sensitivity analyses and scenario simulations were also conducted to ensure these key parameters were determined with sound mechanistic rationale based on current knowledge, including the worst-case scenarios. The verified pralsetinib PBPK model was then applied to predict the effect of other inhibitors and inducers on the PKs of pralsetinib. This work highlights the challenges in understanding DDIs when enzyme-transporter interplay occurs, and demonstrates an important strategy for differentiating enzyme/transporter contributions to enable PBPK predictions for untested scenarios and to inform labeling.

Treatment Trends in Pediatric Trigger Thumb Among Hand Surgeons, Pediatric Orthopedic Surgeons, and Pediatric Hand Surgeons.

BACKGROUND: The appropriate initial management of pediatric trigger thumb (PTT) remains controversial. Some providers advocate for prolonged nonoperative management, whereas others may offer surgical release to provide a reliable and expedient resolution. The goal of this study was to elucidate the practice patterns of surgeons with different fellowship training who treat patients with PTT. We hypothesized that an association between surgeon specialty training and treatment algorithm would be identified. METHODS: A cross-sectional survey was sent to mailing lists of 3 professional organizations whose members represent most providers caring for pediatric hand patients. Respondents were asked their training background and treatment recommendations for several clinical scenarios. Responses were compared across subspecialties. RESULTS: Of the respondents, 444 completed a fellowship in hand surgery, 167 completed a pediatric orthopedic fellowship, and 155 completed an additional congenital hand fellowship. Providers with hand fellowship training were more likely to offer surgical intervention as a first-line treatment for a 3-year-old patient with a flexible trigger thumb than those who completed a pediatric orthopedic fellowship (P = .001), and more likely to offer surgical intervention to a 3-year-old patient with an intermittent (P = .007), painful (P = .015), or locked (P = .012) trigger thumb than those providers who completed additional training in congenital hand surgery. No statistically significant differences in practice patterns were appreciated for children aged 6 and 18 months. CONCLUSION: Variability was appreciated in practice patterns for initial treatment recommendation for a patient presenting with PTT. Subspecialty training does appear to affect treatment recommendations for clinical scenarios involving a 3-year-old patient with PTT, although this trend is not observed when treating younger patients.

Identification of GDC-1971 (RLY-1971), a SHP2 Inhibitor Designed for the Treatment of Solid Tumors.

Protein tyrosine phosphatase SHP2 mediates RAS-driven MAPK signaling and has emerged in recent years as a target of interest in oncology, both for treating with a single agent and in combination with a KRAS inhibitor. We were drawn to the pharmacological potential of SHP2 inhibition, especially following the initial observation that drug-like compounds could bind an allosteric site and enforce a closed, inactive state of the enzyme. Here, we describe the identification and characterization of GDC-1971 (formerly RLY-1971), a SHP2 inhibitor currently in clinical trials in combination with KRAS G12C inhibitor divarasib (GDC-6036) for the treatment of solid tumors driven by a KRAS G12C mutation.

Electrocardiogram Abnormalities Following Diphenhydramine Ingestion: A Case Report.

In the United States, tricyclic antidepressants (TCA) are commonly prescribed to treat psychiatric illnesses and neuropathic pain. This class of antidepressants has been found to cause pathognomonic electrocardiogram (ECG) changes in cases of overdose.1 Specifically, TCA's cause a dominant terminal R wave in aVR and widening of the QRS complex due to their sodium channel blocking effect. Diphenhydramine, better known as Benadryl, is known to disrupt sodium channels in the same manner. In this case report, a 13-year-old female was brought into the emergency department (ED) after attempted suicide by diphenhydramine overdose. The patient presented with palpitations, nausea and confusion. She was agitated, tachycardic and exhibited opsoclonus. An ECG was performed upon the patient's arrival which showed large terminal R waves in aVR along with large S waves in lead I indicating right axis deviation. Given the patient's age and reported ingestion, it was highly suspicious that her symptoms and ECG changes were the result of a sodium channel blockade. Sodium bicarbonate was given, which resulted in notable ECG changes as well as symptomatic improvement. TCA's, and similarly diphenhydramine, have sodium channel blocking properties which can be revealed by performing an ECG. Administration of sodium bicarbonate in the ED has been shown to be a successful treatment by reversing this sodium channel blockade. Topics: Tricyclic antidepressants, diphenhydramine, overdose, sodium channel blockage, sodium bicarbonate administration.

Shared learning from a physiologically based pharmacokinetic modeling strategy for human pharmacokinetics prediction through retrospective analysis of Genentech compounds.

The quantitative prediction of human pharmacokinetics (PK) including the PK profile and key PK parameters are critical for early drug development decisions, successful phase I clinical trials, and the establishment of a range of doses to enable phase II clinical dose selection. Here, we describe an approach employing physiologically based pharmacokinetic (PBPK) modeling (Simcyp) to predict human PK and to validate its performance through retrospective analysis of 18 Genentech compounds for which clinical data are available. In short, physicochemical parameters and in vitro data for preclinical species were integrated using PBPK modeling to predict the in vivo PK observed in mouse, rat, dog, and cynomolgus monkey. Through this process, the in vitro to in vivo extrapolation (IVIVE) was determined and then incorporated into PBPK modeling in order to predict human PK. Overall, the prediction obtained using this PBPK-IVIVE approach captured the observed human PK profiles of the compounds from the dataset well. The predicted Cmax was within 2-fold of the observed Cmax for 94% of the compounds while the predicted area under the curve (AUC) was within 2-fold of the observed AUC for 72% of the compounds. Additionally, important IVIVE trends were revealed through this investigation, including application of scaling factors determined from preclinical IVIVE to human PK prediction for each molecule. Based upon the analysis, this PBPK-based approach now serves as a practical strategy for human PK prediction at the candidate selection stage at Genentech.

Evaluation of bottom-up modeling of the blood-brain barrier to improve brain penetration prediction via physiologically based pharmacokinetic modeling.

Predicting the brain penetration of drugs has been notoriously difficult; however, recently, permeability-limited brain models have been constructed. Lead optimization for central nervous system compounds often focuses on compounds that have low transporter efflux, where passive permeability could be a main driver in determining cerebrospinal fluid (CSF)/brain concentrations. The main objective of this study was to evaluate the translatability of passive permeability data generated from different in vitro systems and its impact on the prediction of human CSF/brain concentrations using physiologically-based pharmacokinetic (PBPK) modeling. In vitro data were generated using gMDCK and parallel artificial membrane permeability assay-blood-brain barrier for comparison and predictions using a quantitative structure-activity relationship model were also evaluated. PBPK modeling was then performed for seven compounds with moderate-high permeability and a range of efflux in vitro, and the CSF/brain mass concentrations and Kpuu were reasonably predicted. This work provides the first step of a promising approach using bottom-up PBPK modeling for CSF/brain penetration prediction to support lead optimization and clinical candidate selection.

Extremity Pressure in Splints Wrapped With an Elastic Bandage Versus Bias Cut Stockinette: An Experimental Model.

BACKGROUND: Many variables affect the pressure caused by splinting or casting. The purpose of this study was to compare pressure underlying a splint wrapped with either an elastic bandage or a bias cut stockinette. METHODS: Thirty-two plaster volar resting splints were applied to a simulated extremity with a saline bag secured to it. A pressure transducer was connected to the saline bag to monitor changes in pressure once splints were applied, and 15 mL increments of saline were added to the bag to simulate swelling. Each dressing type was tested with normal application and tight application. RESULTS: Normal application splints wrapped with either bias cut stockinette or an elastic bandage demonstrated similar initial splint pressures (P = .81). With simulated swelling, splints wrapped with bias cut stockinette demonstrated a 15 mmHg (95% confidence interval [CI], 1.5-28.5) higher average pressure than those wrapped with an elastic bandage (P = .035). Tight application splints with an elastic bandage wrap demonstrated a 46 mmHg (95% CI, 16-77) higher initial splint pressure than those wrapped with bias cut stockinette (P = .009). CONCLUSIONS: Splints wrapped using either an elastic bandage or bias cut stockinette appear to have a similar safety profile, although in cases of excessive swelling, an elastic bandage may provide additional compliance. Tight splint application appears to be more hazardous with the use of an elastic bandage compared with a bias cut stockinette. Further study of the use of elastic bandages and bias cut stockinettes in the clinical setting may be warranted.

Improving the Translation of Organic Anion Transporting Polypeptide Substrates using HEK293 Cell Data in the Presence and Absence of Human Plasma via Physiologically Based Pharmacokinetic Modeling.

Accurately predicting the pharmacokinetics of compounds that are transporter substrates has been notoriously challenging using traditional in vitro systems and physiologically based pharmacokinetic (PBPK) modeling. The objective of this study was to use PBPK modeling to understand the translational accuracy of data generated with human embryonic kidney 293 (HEK293) cells overexpressing the hepatic uptake transporters organic anion transporting polypeptide (OATP) 1B1/3 with and without plasma while accounting for transporter expression. Models of four OATP substrates, two with low protein binding (pravastatin and rosuvastatin) and two with high protein binding (repaglinide and pitavastatin) were explored, and the OATP in vitro data generated in plasma incubations were used for a plasma model, and in buffer incubations for a buffer model. The pharmacokinetic parameters and concentration-time profiles of pravastatin and rosuvastatin were similar and well predicted (within 2-fold of observed values) using the plasma and buffer models without needing an empirical scaling factor, whereas the dispositions of the highly protein bound repaglinide and pitavastatin were more accurately simulated with the plasma models than the buffer models. This work suggests that data from HEK293 overexpressing transporter cells corrected for transporter expression represent a valid approach to improve bottom-up PBPK modeling for highly protein bound OATP substrates with plasma incubations and low protein binding OATP substrates with or without plasma incubations. SIGNIFICANCE STATEMENT: This work demonstrates the bottom-up approach of using in vitro data directly without employing empirical scaling factors to predict the intravenous pharmacokinetic (PK) profiles reasonably well for four organic anion transporting polypeptide (OATP) substrates. Based on these results, using HEK293 overexpressing cells, examining the impact of plasma for highly bound compounds, and incorporating transporter quantitation for the lot in which the in vitro data were generated represents a valid approach to achieve more accurate prospective PK predictions for OATP substrates.

Examination of Physiologically-Based Pharmacokinetic Models of Rosuvastatin.

Physiologically-based pharmacokinetic (PBPK) modeling is increasingly used to predict drug disposition and drug-drug interactions (DDIs). However, accurately predicting the pharmacokinetics of transporter substrates and transporter-mediated DDIs (tDDIs) is still challenging. Rosuvastatin is a commonly used substrate probe in DDI risk assessment for new molecular entities (NMEs) that are potential organic anion transporting polypeptide 1B or breast cancer resistance protein transporter inhibitors, and as such, several rosuvastatin PBPK models have been developed to try to predict the clinical DDI and support NME drug labeling. In this review, we examine five representative PBPK rosuvastatin models, discuss common challenges that the models have come across, and note remaining gaps. These shared learnings will help with the continuing efforts of rosuvastatin model validation, provide more information to understand transporter-mediated drug disposition, and increase confidence in tDDI prediction.

Changes in Organic Anion Transporting Polypeptide Uptake in HEK293 Overexpressing Cells in the Presence and Absence of Human Plasma.

Generating accurate in vitro data is crucial for in vitro to in vivo extrapolation and pharmacokinetic predictions. The use of human embryonic kidney (HEK) 293 cells overexpressing organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 in protein-free buffer and 100% human plasma incubations was explored for the uptake of four OATP substrates: pravastatin, rosuvastatin, repaglinide, and pitavastatin. Differences were observed for each parameter [unbound Michaelis constant (K m,u), V max, intrinsic clearance (CLint), and unbound passive diffusion Pdif,u] obtained from the buffer and plasma incubations in both cells, and the fold differences were greatest for the highly protein bound compounds. The fold change in K m,u values ranged from 1.91 to 619, and the fold change in V max values ranged from 1.22 to 97.4. As a result, in both cells, the CLint values generated in the plasma incubations were higher by 0.762- to 31.7-fold than the values generated in the protein-free buffer. The passive diffusion was also higher in the plasma incubations for all four compounds, with a fold difference range of 1.73-23.4. These shifts in the presence and absence of human plasma suggest that plasma proteins may play a role in both the active uptake and passive diffusion processes. The results also support the idea of a transporter-induced protein-binding shift, where high protein binding may not limit the uptake of compounds that have high affinity for transporters. The addition of plasma to incubations leading to higher CLint values for transporter substrates helps mitigate the underprediction commonly noted with in vitro to in vivo extrapolation. SIGNIFICANCE STATEMENT: The current investigation brings a new perspective on how to mitigate the underprediction commonly noted with in vitro to in vivo extrapolation for OATP substrates by using HEK293 cells overexpressing OATP1B1 and OATP1B3. It also supports the idea of a transporter-induced protein-binding shift, where high protein binding may not limit the uptake of compounds that have high affinity for transporters.

In Vitro-In Vivo Inaccuracy: The CYP3A4 Anomaly.

When predicting hepatic clearance using in vitro to in vivo extrapolation (IVIVE), microsomes or hepatocytes are commonly used. Here, we examine intrinsic clearance values and IVIVE results in human hepatocytes and microsomes for compounds metabolized by a variety of enzymes. The great majority of CYP3A4 substrates examined had higher intrinsic clearance values in microsomes compared with hepatocytes, whereas the values were more similar between the two incubations for substrates of other enzymes. We hypothesize that this may be due to interplay between CYP3A4 and the efflux transporter P-glycoprotein, as they have been shown to exhibit coordinated regulation. When examining the prediction accuracy for substrates of other enzymes between microsomes and hepatocytes, average fold errors as well as overall error were similar, demonstrating once again that IVIVE methods are not adequately defined and understood. SIGNIFICANCE STATEMENT: For CYP3A4 substrates, microsomes give markedly higher predictive in vitro to in vivo extrapolation than for other metabolic enzymes, which is not found for hepatocytes. We hypothesize that this could be a result of CYP3A4-P-glycoprotein interplay or coordinated regulation in hepatocytes that would not be observed in microsomes.

Use of an Accelerated Discharge Pathway in Patients With Severe Cerebral Palsy Undergoing Posterior Spinal Fusion for Neuromuscular Scoliosis.

BACKGROUND: Implementation of a coordinated multidisciplinary postoperative pathway has been shown to reduce length of stay after posterior spinal fusion (PSF) for adolescent idiopathic scoliosis. This study sought to compare the outcomes of nonambulatory cerebral palsy (CP) patients treated with PSF and cared for using an accelerated discharge (AD) pathway with those using a more traditional discharge (TD) pathway. METHODS: A total of 74 patients with Gross Motor Function Classification System (GMFCS) class 4/5 CP undergoing PSF were reviewed. Thirty consecutive patients were cared for using a TD pathway, and 44 patients were subsequently treated using an AD pathway. The cohorts were then evaluated for postoperative complications and length of stay. RESULTS: Length of stay (LOS) was 19% shorter in patients managed with the AD pathway (AD 4.0 days [95% CI 2.5-5.5] vs. TD 4.9 days [95% CI 3.5-6.3], p = .01). There was no difference between groups with respect to age at surgery, GMFCS class, preoperative curve magnitude, pelvic obliquity, kyphosis, postoperative curve correction, fusion to the pelvis, or length of fusion between groups. Length of stay remained significantly shorter in the AD group by 0.9 days when controlling for estimated blood loss (EBL) and length of surgery. Complication rates trended lower in the AD group (33% AD vs. 52% TD, p = .12), including pulmonary complications (21% AD vs. 38% TD, p = .13). There was no significant difference in wound complications, return to the operating room, or medical readmissions between groups. CONCLUSIONS: Adoption of a standardized postoperative pathway reduced LOS by 19% in nonambulatory CP patients. Overall, complications, including pulmonary, trended lower in the AD group. Early discharge appears to be possible in this challenging patient population. Although the AD pathway may not be appropriate for all patients, the utility of the AD pathway in optimizing care for more routine PSF for this patient subset appears to be worthwhile. LEVEL OF EVIDENCE: Level III, therapeutic.

How Transporters Have Changed Basic Pharmacokinetic Understanding.

The emergence and continued evolution of the transporter field has caused re-evaluation and refinement of the original principles surrounding drug disposition. In this paper, we emphasize the impact that transporters can have on volume of distribution and how this can affect the other major pharmacokinetic parameters. When metabolic drug-drug interactions or pharmacogenomic variance changes the metabolism of a drug, the volume of distribution appears to be unchanged while clearance, bioavailability, and half-life are changed. When transporters are involved in the drug-drug interactions or pharmacogenomic variance, the volume of distribution can be markedly affected causing counterintuitive changes in half-life. Cases are examined where a volume of distribution change is significant enough that although clearance decreases, half-life decreases. Thus, drug dosing decisions must be made based on CL/F changes, not half-life changes, as such volume of distribution alterations will also influence the half-life results.

An Exceptionally Potent Inhibitor of Human CD73.

We recently reported the initiation of a Phase I clinical trial with AB680, a potent human CD73 inhibitor, being developed for the treatment of solid tumors (NCT03677973). We undertook a detailed kinetic analysis of the interaction between human CD73 and AB680 to determine the mode of inhibition. We found AB680 to be a reversible, slow-onset competitive inhibitor of human CD73 with a Ki of 5 pM. Clinical candidates of this potency are uncommon and deserve special consideration during lead optimization.

Interlaboratory Variability in Human Hepatocyte Intrinsic Clearance Values and Trends with Physicochemical Properties.

PURPOSE: To examine the interlaboratory variability in CLint values generated with human hepatocytes and determine trends in variability and clearance prediction accuracy using physicochemical and pharmacokinetic parameters. METHODS: Data for 50 compounds from 14 papers were compiled with physicochemical and pharmacokinetic parameter values taken from various sources. RESULTS: Coefficients of variation were as high as 99.8% for individual compounds and variation was not dependent on the number of prediction values included in the analysis. When examining median values, it appeared that compounds with a lower number of rotatable bonds had more variability. When examining prediction uniformity, those compounds with uniform in vivo underpredictions had higher CLint, in vivo values, while those with non-uniform predictions typically had lower CLint, in vivo values. Of the compounds with uniform predictions, only a small number were uniformly predicted accurately. Based on this limited dataset, less lipophilic, lower intrinsic clearance, and lower protein binding compounds yield more accurate clearance predictions. CONCLUSIONS: Caution should be taken when compiling in vitro CLint values from different laboratories as variations in experimental procedures (such as extent of shaking during incubation) may yield different predictions for the same compound. The majority of compounds with uniform in vitro values had predictions that were inaccurate, emphasizing the need for a better mechanistic understanding of IVIVE. The non-uniform predictions, often with low turnover compounds, reaffirmed the experimental challenges for drugs in this clearance range. Separating new chemical entities by lipophilicity, intrinsic clearance, and protein binding may help instill more confidence in IVIVE predictions.

Understanding drug-drug interaction and pharmacogenomic changes in pharmacokinetics for metabolized drugs.

Here we characterize and summarize the pharmacokinetic changes for metabolized drugs when drug-drug interactions and pharmacogenomic variance are observed. Following multiple dosing to steady-state, oral systemic concentration-time curves appear to follow a one-compartment body model, with a shorter rate limiting half-life, often significantly shorter than the single dose terminal half-life. This simplified disposition model at steady-state allows comparisons of measurable parameters (i.e., area under the curve, half-life, maximum concentration and time to maximum concentration) following drug interaction or pharmacogenomic variant studies to be utilized to characterize whether a drug is low versus high hepatic extraction ratio, even without intravenous dosing. The characteristics of drugs based on the ratios of area under the curve, maximum concentration and half-life are identified with recognition that volume of distribution is essentially unchanged for drug interaction and pharmacogenomic variant studies where only metabolic outcomes are changed and transporters are not significantly involved. Comparison of maximum concentration changes following single dose interaction and pharmacogenomic variance studies may also identify the significance of intestinal first pass changes. The irrelevance of protein binding changes on pharmacodynamic outcomes following oral and intravenous dosing of low hepatic extraction ratio drugs, versus its relevance for high hepatic extraction ratio drugs is re-emphasized.

In Vitro-In Vivo Extrapolation and Hepatic Clearance-Dependent Underprediction.

Accurately predicting the hepatic clearance of compounds using in vitro to in vivo extrapolation (IVIVE) is crucial within the pharmaceutical industry. However, several groups have recently highlighted the serious error in the process. Although empirical or regression-based scaling factors may be used to mitigate the common underprediction, they provide unsatisfying solutions because the reasoning behind the underlying error has yet to be determined. One previously noted trend was intrinsic clearance-dependent underprediction, highlighting the limitations of current in vitro systems. When applying these generated in vitro intrinsic clearance values during drug development and making first-in-human dose predictions for new chemical entities though, hepatic clearance is the parameter that must be estimated using a model of hepatic disposition, such as the well-stirred model. Here, we examine error across hepatic clearance ranges and find a similar hepatic clearance-dependent trend, with high clearance compounds not predicted to be so, demonstrating another gap in the field.

The Presence of a Transporter-Induced Protein Binding Shift: A New Explanation for Protein-Facilitated Uptake and Improvement for In Vitro-In Vivo Extrapolation.

Accurately predicting hepatic clearance is an integral part of the drug-development process, and yet current in vitro to in vivo (IVIVE) extrapolation methods yield poor predictions, particularly for highly protein-bound transporter substrates. Explanations for error include inaccuracies in protein-binding measurements and the lack of recognition of protein-facilitated uptake, where both unbound and bound drug may be cleared, violating the principles of the widely accepted free drug theory. A new explanation for protein-facilitated uptake is proposed here, called a transporter-induced protein binding shift High-affinity binding to cell-membrane proteins may change the equilibrium of the nonspecific binding between drugs and plasma proteins, leading to greater cellular uptake and clearance than currently predicted. The uptake of two lower protein-binding organic anion transporting polypeptide substrates (pravastatin and rosuvastatin) and two higher binding substrates (atorvastatin and pitavastatin) were measured in rat hepatocytes in incubations with protein-free buffer versus 100% plasma. Decreased unbound K m values and increased intrinsic clearance values were seen in the plasma incubations for the highly bound compounds, supporting the new hypothesis and mitigating the IVIVE underprediction previously seen for highly bound transporter substrates.

The Extended Clearance Concept Following Oral and Intravenous Dosing: Theory and Critical Analyses.

PURPOSE: To derive the theoretical basis for the extended clearance model of organ elimination following both oral and IV dosing, and critically analyze the approaches previously taken. METHODS: We derived from first principles the theoretical basis for the extended clearance concept of organ elimination following both oral and IV dosing and critically analyzed previous approaches. RESULTS: We point out a number of critical characteristics that have either been misinterpreted or not clearly presented in previously published treatments. First, the extended clearance concept is derived based on the well-stirred model. It is not appropriate to use alternative models of hepatic clearance. In analyzing equations, clearance terms are all intrinsic clearances, not total drug clearances. Flow and protein binding parameters should reflect blood measurements, not plasma values. In calculating the AUCR-factor following oral dosing, the AUC terms do not include flow parameters. We propose that calculations of AUCR may be a more useful approach to evaluate drug-drug and pharmacogenomic interactions than evaluating rate-determining steps. Through analyses of cerivastatin and fluvastatin interactions with cyclosporine we emphasize the need to characterize volume of distribution changes resulting from transporter inhibition/induction that can affect rate constants in PBPK models. Finally, we note that for oral doses, prediction of systemic and intrahepatic drug-drug interactions do not require knowledge of fu,H or Kp,uu for substrates/victims. CONCLUSIONS: The extended clearance concept is a powerful tool to evaluate drug-drug interactions, pharmacogenomic and disease state variance but evaluating the AUCR-factor may provide a more valuable approach than characterizing rate-determining steps.

A workforce in jeopardy: identifying the challenges of ensuring a sustainable advanced HIV nursing workforce.

BACKGROUND: HIV services in England face substantial challenges arising from financial pressures and changes to commissioning. A sustainable HIV specialist nursing workforce will be vital to enable them to respond to those challenges. AIMS: This paper examines the current workforce situation in HIV services across the country. METHODS: This mixed-method study involved semi-structured interviews with 19 key stakeholders and with 44 nurses/physicians from 21 purposively selected HIV services across England. Data were interpreted using a framework analysis approach. RESULTS: 'Building a career in HIV nursing' identified problems associated with retention and recruitment. Changes in commissioning are disrupting common career routes from sexual health to HIV nursing, and a perceived lack of a clear career pathway was seen as a barrier to recruitment. 'Developing a specialist workforce' explored the professional development of the current workforce, which was hampered by poor access to funding or study time for advanced study and the absence of an HIV-specific advanced nursing qualification. CONCLUSIONS: The HIV nursing workforce, which provides an increasing proportion of HIV care, is facing serious recruitment and retention challenges. A strategic approach to workforce development and training is essential to overcome systemic barriers and secure the next generation of skilled practitioners.