Remembering the null hypothesis when searching for brain sex differences

Human brain sex differences have fascinated scholars for centuries and become a key focus of neuroscientists since the dawn of MRI. We recently published a major review in Neuroscience and Biobehavioral Reviews showing that most male-female brain differences in humans are small and few have been reliably replicated. Although widely cited, this work was the target of a critical Commentary by DeCasien et al. (Biol Sex Differ 13:43, 2022). In this response, I update our findings and confirm the small effect sizes and pronounced scatter across recent large neuroimaging studies of human sex/gender difference. Based on the sum of data, neuroscientists would be well-advised to take the null hypothesis seriously: that men and women's brains are fundamentally similar, or "monomorphic". This perspective has important implications for how we study the genesis of behavioral and neuropsychiatric gender disparities.

Why and How to Account for Sex and Gender in Brain and Behavioral Research

Long overlooked in neuroscience research, sex and gender are increasingly included as key variables potentially impacting all levels of neurobehavioral analysis. Still, many neuroscientists do not understand the difference between the terms "sex" and "gender," the complexity and nuance of each, or how to best include them as variables in research designs. This TechSights article outlines rationales for considering the influence of sex and gender across taxa, and provides technical guidance for strengthening the rigor and reproducibility of such analyses. This guidance includes the use of appropriate statistical methods for comparing groups as well as controls for key covariates of sex (e.g., total intracranial volume) and gender (e.g., income, caregiver stress, bias). We also recommend approaches for interpreting and communicating sex- and gender-related findings about the brain, which have often been misconstrued by neuroscientists and the lay public alike.

Breaking the binary: Gender versus sex analysis in human brain imaging

Despite decades of pursuit, human brain imaging has yet to uncover clear neural correlates of male-female behavioral differences. Given that such behavior does not always align with sex categories, we argue that neuroimaging research may find more success by partitioning subjects along nonbinary gender attributes in addition to sex. We review the handful of studies that have done this, several of which find as good or better association between brain measures and "gender" as they do with "sex." Recent advances in operationalizing "gender" as a multidimensional variable should facilitate such studies, along with discovery-based approaches that mine brain imaging data for gender-associated attributes, independent of sex.

How hype and hyperbole distort the neuroscience of sex differences

Mind your language! This Perspective article points out that the source of misreporting in the field of sex/gender differences can often be found in the unjustified overstatements used by researchers themselves in reporting their findings; caution is needed when addressing differences between human beings, and hype and hyperbole should be avoided.

Consensus Parameter: Research Methodologies to Evaluate Neurodevelopmental Effects of Pubertal Suppression in Transgender Youth

Purpose: Pubertal suppression is standard of care for early pubertal transgender youth to prevent the development of undesired and distressing secondary sex characteristics incongruent with gender identity. Preliminary evidence suggests pubertal suppression improves mental health functioning. Given the widespread changes in brain and cognition that occur during puberty, a critical question is whether this treatment impacts neurodevelopment.

Methods: A Delphi consensus procedure engaged 24 international experts in neurodevelopment, gender development, puberty/adolescence, neuroendocrinology, and statistics/psychometrics to identify priority research methodologies to address the empirical question: is pubertal suppression treatment associated with real-world neurocognitive sequelae? Recommended study approaches reaching 80% consensus were included in the consensus parameter.

Results: The Delphi procedure identified 160 initial expert recommendations, 44 of which ultimately achieved consensus. Consensus study design elements include the following: a minimum of three measurement time points, pubertal staging at baseline, statistical modeling of sex in analyses, use of analytic approaches that account for heterogeneity, and use of multiple comparison groups to minimize the limitations of any one group. Consensus study comparison groups include untreated transgender youth matched on pubertal stage, cisgender (i.e., gender congruent) youth matched on pubertal stage, and an independent sample from a large-scale youth development database. The consensus domains for assessment includes: mental health, executive function/cognitive control, and social awareness/functioning.

Conclusion: An international interdisciplinary team of experts achieved consensus around primary methods and domains for assessing neurodevelopmental effects (i.e., benefits and/or difficulties) of pubertal suppression treatment in transgender youth.

Meta-analysis reveals a lack of sexual dimorphism in human amygdala volume

The amygdala plays a key role in many affective behaviors and psychiatric disorders that differ between men and women. To test whether human amygdala volume (AV) differs reliably between the sexes, we performed a systematic review and meta-analysis of AVs reported in MRI studies of age-matched healthy male and female groups. Using four search strategies, we identified 46 total studies (58 matched samples) from which we extracted effect sizes for the sex difference in AV. All data were converted to Hedges g values and pooled effect sizes were calculated using a random-effects model. Each dataset was further meta-regressed against study year and average participant age. We found that uncorrected amygdala volume is about 10% larger in males, with pooled sex difference effect sizes of g=0.581 for right amygdala (κ=28, n=2022), 0.666 for left amygdala (κ=28, n=2006), and 0.876 for bilateral amygdala (κ=16, n=1585) volumes (all p values < 0.001). However, this difference is comparable to the sex differences in intracranial volume (ICV; g=1.186, p<.001, 11.9% larger in males, κ=11) and total brain volume (TBV; g=1.278, p<0.001, 11.5% larger in males, κ=15) reported in subsets of the same studies, suggesting the sex difference in AV is a product of larger brain size in males. Among studies reporting AVs normalized for ICV or TBV, sex difference effect sizes were small and not statistically significant: g=0.171 for the right amygdala (p=0.206, κ=13, n=1560); 0.233 for the left amygdala (p=0.092, κ=12, n=1512); and 0.257 for bilateral volume (p=0.131, κ=5, n=1629). These values correspond to less than 0.1% larger corrected right AV and 2.5% larger corrected left AV in males compared to females. In summary, AV is not selectively enhanced in human males, as often claimed. Although we cannot rule out subtle male-female group differences, it is not accurate to refer to the human amygdala as "sexually dimorphic."

Steady-state pharmacokinetic comparison of a new, once-daily, extended-release morphine formulation (Morphelan™) and OxyContin® twice daily

Context. Morphelan2 (morphine sulfate extended release), a new, once-daily, oral morphine sulfate formulation, releases morphine in a dual-release manner, providing an initial rapid delivery while maintaining therapeutic morphine concentrations throughout a 24-hour period with minimal fluctuation.

Objective. The primary objective of this study was to compare steady-state pharmacokinetic (PK) profiles of morphine and oxycodone following administration of 60 mg of Morphelan, once daily and 20 mg of OxyContin1 (oxycodone hydrochloride controlled release) twice daily, respectively.

Design, Setting, Participants, Intervention. Thirty-five healthy subjects were randomized to receive Morphelan once daily, or OxyContin twice daily for 7 days in this multiple-dose, open-label, parallel design study.

Measures. Blood samples were obtained on day 7 to determine differences, if any, in the PK parameters that characterize the ability of the formulations to control the release of drug. Safety data were collected throughout the study.

Results. Thirty-two subjects completed the study; 31 subjects were eligible for PK evaluation. The time to reach and maintain threshold concentrations of 50% or 75% of maximum was comparable for Morphelan and OxyContin. However, in comparison to OxyContin twice daily, once-daily Morphelan provided a statistically significantly reduced peak-to-trough fluctuation (%FI = 91% vs 77%, respectively, P= 0.05) and a lower number of fluctuations (2 vs 1, respectively) over a 24-hour period.

Conclusions. Once-daily Morphelan provided an initial rapid release of opioid reaching and maintaining threshold concentrations of 50% and 75% of maximum comparable to OxyContin twice daily. However, Morphelan provided more consistent opioid concentrations throughout a 24-hour period, with less frequent dosing.

Steady-state pharmacokinetic comparison of a new, extended-release, once-daily morphine formulation, Avinza, and a twice-daily controlled-release morphine formulation in patients with chronic moderate-to-severe pain

Extended-release morphine formulations are widely used in the management of chronic pain. Avinza (morphine sulfate extended-release [MSER, Morphelan]) is a new, once-a-day, extended-release morphine formulation designed to reach target concentrations rapidly and maintain concentrations throughout a 24-hour period. The primary objective of this study was to compare the 24-hour steady-state pharmacokinetic (PK) profiles of morphine and its metabolites (morphine-6-glucuronide [M6G] and morphine-3-glucuronide [M3G]) following ingestion of MSER once-a-day and MS Contin (controlled-release morphine sulfate [CRM]) twice-a-day in patients with chronic moderate-to-severe pain. Ten patients with chronic moderate-to-severe pain were recruited into an open-label, multiple-dose, nonrandomized, two-period, single-center study. All patients were stabilized for a minimum of 7 days on a twice-daily dose of CRM associated with an optimal balance between pain control and side effects. Patients were then switched to the closest equivalent once-daily dose of MSER for a minimum of 10 days. Twenty-four hour steady-state PK profiles were obtained on the last day of each treatment period and additional clinical and safety assessments were performed. PK data were normalized to a 100-mg total daily dose prior to statistical analysis. Nine of the 10 patients completed the entire study. MSER and CRM demonstrated similar bioavailability (AUC) of morphine and its metabolites. Compared to CRM, MSER demonstrated a 19% lower maximum concentration (C(max)), a 66% higher minimum concentration (C(min)), and a 44% lower peak-to-trough fluctuation (%FI) over the 24-hour period. In addition, MSER maintained concentrations above 50% and 75% of the C(max) longer than CRM. Clinical efficacy and safety were comparable for MSER and CRM. Once-daily MSER approaches maximum morphine concentration more quickly, approximates maximum concentration longer, and demonstrates less fluctuation in morphine concentration during a 24-hour period than CRM dosed twice daily. The pharmacodynamic implications of this profile deserve further study.

Efficacy and safety of a once-daily morphine formulation in chronic, moderate-to-severe osteoarthritis pain: results from a randomized, placebo-controlled, double-blind trial and an open-label extension trial

A randomized, 4-week, double-blind trial followed by an open-label extension trial assessed the efficacy and safety of a once-daily, extended-release morphine formulation (Avinza (previously referred to as Morphelan)) in 295 patients with chronic, moderate-to-severe osteoarthritis pain who had failed to obtain adequate pain relief with NSAIDs and acetaminophen. Participants received one of four treatments: Avinza 30 mg once daily (QAM or QPM), MS Contin(R) 15 mg twice daily, or placebo twice daily. Patients (n =181) received Avinza QAM or QPM during the 26-week open-label extension trial and could increase their dose to optimize pain control. Avinza and MS Contin reduced pain and improved several sleep measures versus placebo. Analgesic efficacy was comparable between Avinza and MS Contin; however, Avinza QAM demonstrated greater improvements in overall quality of sleep. The most common adverse events were constipation and nausea. The majority of AEs occurred at a similar incidence among the active treatment groups.

Pharmacokinetic evaluation of a sprinkle-dose regimen of a once-daily, extended-release morphine formulation

BACKGROUND

Morphine sulfate extended-release (MSER) uses a drug-delivery technology that allows once-daily dosing. It is possible to open the MSER capsule and sprinkle the contents on soft food, a potentially useful alternative to the intact capsule in patients who have difficulty swallowing.

OBJECTIVE

This study compared the bioavailability of MSER and its metabolites morphine-3-glucuronide and morphine-6-glucuronide after administration of MSER in a sprinkle-dose regimen relative to an intact capsule swallowed whole.

METHODS

This was a randomized, open-label, single-dose, crossover study, with a 7-day washout period between the 2 dosing days. Healthy volunteers were randomized to receive an intact 60-mg MSER capsule swallowed whole or the contents of a 60-mg MSER capsule sprinkled on applesauce. Blood samples were collected and analyzed for concentrations of morphine and its active glucuronide metabolites. Pharmacokinetic (PK) parameters were calculated and bioequivalence assessed. Bioequivalence was concluded if the 90% CIs of the ratio of log-transformed values for maximum concentration (Cmax) and area under the plasma concentration-time curve (AUC) were within 80% to 125%.

RESULTS

Of 30 subjects enrolled, 28 completed the study and were eligible for PK evaluation. Two subjects were withdrawn for reasons unrelated to study treatment. The plasma concentration-time profiles of morphine and its metabolites were superimposable after administration of the 2 regimens. Cmax and total systemic exposure-based on AUC from time 0 to the last quantifiable concentration (AUC(last)) and AUC from time 0 to infinity (AUC(infinity))-were comparable between treatments. The 90% CIs for morphine AUC(last), AUC, and Cmax ratios were 98 to 109, 96 to 106, and 95 to 117, respectively. Similar 90% CIs were obtained for the morphine metabolites.

CONCLUSION

In this study in healthy volunteers, sprinkling the entire contents of an MSER capsule onto applesauce and swallowing without chewing was bioequivalent to swallowing an intact MSER capsule.

The pharmacokinetics, pharmacodynamics, safety and tolerability following 7 days daily oral treatment with NN703 in healthy male subjects

The aim of the present study was to assess the safety, pharmacokinetics and pharmacodynamics (including specificity) of NN703 (tabimorelin), a growth hormone (GH) secretagogue, in healthy male subjects following treatment for 7 days once-daily. This was a randomized, double-blind and placebo-controlled study with four active dose levels: 1.71, 3.0, 4.5 and 6.86 mg/kg body weight. There was a dose-related increase for GH area under the curve (AUC) (0-12 h) and GH C(max)(0--12 h); these were significantly higher on both days 1 and 7 as compared with placebo treatment (P = 0.04 to P< 0.0001); however, an overall significant decrease in GH release was found from day 1 to day 7 (P< 0.001). Insulin-like growth factor-I (IGF-I) and IGF binding protein 3 (IGFBP-3) increased at all dose levels (including placebo); however, a significantly higher increase as compared with placebo treatment was observed at the three highest dose levels for IGF-I (P = 0.04--0.0006) and at the highest dose level for IGFBP-3 (P = 0.03). There was no statistically significant increase in AUC (0-5 h) for follicle-stimulating hormone, luteinizing hormone and cortisol between active and placebo treatment for day 1 or 7. On day 1 only, a statistically significant increase in AUC (0--5 h) was found for prolactin at 1.71 and 6.86 mg/kg (P< 0.05), for thyroid-stimulating hormone (TSH) at 3.0 mg/kg (P< 0.01) and for adrenocorticotrophic hormone (ACTH) at 4.5 mg/kg (P< 0.05); however, no dose--response relationship was observed for TSH or ACTH. In addition, a statistically significant decrease in AUC (0--5 h) for ACTH (3.0 and 6.86 mg/kg) and cortisol (1.71 mg/kg) was observed on day 7 (P< 0.05). Thus, NN703 is a promising candidate for treatment of absolute or relative GH deficiency.